Psyker

Good thing Games Workshop has an extremely liberal interpretation of the word 'psychic'. Magic IN SPEHSS! Eventually, all humans will be doing this for lulz against man's enemies. Or turning ourselves inside-out by accident yet not dying. Hopefully, New Man wields the power of his Warp soul instead of just Warp or we’re all fucked.

"Whoever fights monsters should see to it that in the process he does not become a monster."

– Friedrich Nietzsche

"We always vilify what we don't understand."

– Nenia Campbell

"WITNESS YOUR DOOOOM!"

– an Imperial Guard Psyker about to blast some heretics and/or xenos with Lightning Arc

A psyker, in the Warhammer 40,000 universe, is an individual with psychic powers capable of accessing the Warp. This is both a blessing and a curse; though psykers are often able to achieve spectacular feats with their abilities, the Warp is a perilous place, and whenever a psyker uses their abilities there is a chance that they will be subject to the Perils of the Warp. Death is one of the milder results of this; extremely unlucky psykers might actually be tortured in countless ways by daemons and used as a gateway from the Warp into the physical world. Luckily, in the ranks of the Imperial Guard, where there's a psyker, there's usually a Commissar ready to unleash the power of Blam.

Types of Psyker

The different races of the setting perceive psykers in different ways.

Imperium

The Imperium of Man regards most psykers as extremely dangerous, because these are people with scary powers, and the influence of the Warp tends not to let them stay sane for very long, but hey, wizards are real, coolHERESY! BLAM. There is also the threat of Chaos possessing them and summoning daemons...or having the daemons just eat them. Thus, the Inquisition hunts them down so they can either be sacrificed to feed the Empra or sanctioned and made to serve the Imperium (under an inquisitor's watchful eye). Such psykers appear as a unit in the Dawn of War games and like to scream incomprehensible one-liners so mind-numbingly loud that they may very well actually make your head explode. Space Marines make their psykers into Librarians: in addition to psychic support on the battlefield, they compile records in the Librarium, assist in battlefield communications, and test their brothers for Chaotic contamination. Imperial Guard Sanctioned Psykers combine psychic powers with Standard Issue Balls of Steel that every Imperial Guard member is issued upon conscription along with a lasgun. In addition, they have a nasty reputation for being BLAMMED by Commissars. "Rogue Psykers" is the official designation for any psyker who hasn't been collared by the Imperium yet and are often hated by more puritan worlds, in extreme cases routinely being hunted down and killed. On most worlds being sanctioned (assuming you survive and don't mind the side effects) gives an air of authority and orthodoxy to an otherwise hated thing; people still hate you, but instead of treating you like a particularly verbose bit of dung, now you're hated the way tax collectors are hated. Many in the Imperium outside of the rabble view sanctioned psykers in an ethereal light, considering them sages or wizards (as in wise men/Gandalf), perhaps because their powers grant them a very different perspective and insight into pretty much anything. Astropaths are beyond such venom, partly because they are less prone to possession and mostly because they are a planet's only way of communicating with the greater Imperium. Also being soul-bound to the Emperor himself probably makes them seem like prophets or oracles in some way to the average citizen. People still feel uneasy around them, but even Black Templars have to cope with them and make nice faces. However, the Emperor was is a psyker, so the next time some idiot tells you that Psykers are heresy, use this little fact on their fa--*BLAM* BLASPHEMY!. Well, actually not, since for an average citizen of the Imperium the Emperor is God, so His wielding godly powers in the glory days before His ascension is actually the core tenet of the Imperial Cult and proof of his divinity. Not so for mere mortal psykers, who are rightly considered unholy and corruptible as opposed to a literal god who obviously can do things like that easy peasy, which he did demonstrate that he could. Oh, you mortally wounded me through my favored lab rat? Let me just erase your collective avatar out of existence in one shot.

Humanity, being the stars of the show, have arguably the greatest psychic potential overall (because of course they do, which is really damn bad in this setting), but with barely any control over it (fire and lightning yay!), possibly evolving (which could take countless centuries or even millennia to take place, even if they can survive it) into a fully psychic race which the Emperor himself has said may very well equal or even possibly surpass even the Eldar at their peak, although with the loss of the Webway project even the Emperor admits that this path is pretty much now closed to humanity. Most likely he meant a gradual ascension instead of the Oh Shit happening now as the energies of the Webway logically would nourish psychic potential slowly whereas otherwise human potential is spastic and often results in daemonic possession or outright invasion. No wonder his ideal humans are emotionless and autistic (no, seriously, the Custodes are arguably genuinely autistic) as that is safest for a psychic species, especially to not go the way of the Eldar. Becoming a psychic race doesn't suddenly make you god-tier, it just makes you psychic; the Eldar are a full-blown psychic race and they aren't exactly doing brilliantly. To be fair, they’re not even trying to capitalize on their psychic potential most of the time, especially the ways psychic power could easily connect to the aspects of Khaine their warriors emulate. On the other-hand, that’s probably exactly why they don’t do that.

• Like all of his interactions with others, anything that the Emperor says should be treated as a possible falsehood (he's a bit of a compulsive liar and manipulator, but even when he’s honest, his power actively causes people to hear things in ways that make sense to them, even if what they hear isn’t close to what he said). His whole speech to Ra about humanity's possible psychic might may well have been just what the Emperor wanted Ra to hear in order to prepare him to make his sacrifice at the end of the book. But we do know that the lore has loooong held the Emperor’s belief humanity will become like him. AKA “New Man”. Even Rogue Trader had the Inquisition’s true purpose be to guide humanity’s ascension into a fully psychic species.

On the bright side, the Psychic Awakening has shown that faith serves as a great focus for controlling psychic might and affecting the Warp in a positive way through that focus of belief and spiritual strength. Bet the Eldar are foaming at the mouth that man’s much mocked fanaticism actually acts as anti-Chaos space magic.

Ironically, humanity’s strong connection to Chaos as the species that caused the gods to awaken might be the cause of our destined rise as a powerfully psychic species. Which is, uh, not a good sign if true.

Humanity has managed to produce some of the strongest Psykers in the history of the Galaxy but most didn't come into being through natural means:

• The Emperor is obviously the first that springs to mind as he is unquestionably the strongest mortal psyker that has yet been seen, unless you count the Old Ones (Emps could be an Old One masquerading as a human, but is probably way beyond an Old One seeing as he beat the shit out of one of the most powerful C'tan by a whole lot). The question is however: is he even human? There are many theories surrounding the Emperor's origins and his exact nature. Was he born or is he a construct from the Dark Age of Technology that believes itself to be human? Is he the Warp-manifested core of humanity's past, present, and future manifested in Man's time of greatest need and tumultuous Warp? If we take the whole Shaman origin as fact, then his psychic might is due to the large number of psychic beings merging their minds into one, which is certainly not the norm (strangely something similar happened to Ephrael Stern). Even if we assume he's a single entity that happens to be immortal, there is the whole deal with the Chaos gods which may or may not have boosted his already considerable power (and yes, it is stated that he needed a ship to get there, but didn't need one to get back). The Emperor is clear proof that, whatever he might be, the most powerful psyker in existence has indisputably allied itself with humanity.
• Magnus is a Primarch and should in no way be considered entirely human. He, like all his kind, are artificial beings created through Warp shenanigans and advanced bio-engineering. There is nothing natural about him or his psychic might. Above and beyond even high-grade transhumans, a Primarch is a literal demigod perfected through soul and gene engineering. This translates to Psychic ability as well.
• Malcador is the only truly natural (depending upon what happens with his backstory once it is finally revealed) human standout amongst the big names. He may play second fiddle to the likes of the abominations that are the Emperor*BLAM* BLASPHEMY! and Magnus, but he is still the strongest "human" psyker on the list. Natural humans are never going to be able to compete with the gods of the Warp, but a race of potential Malcadors doesn't sound too shabby, does it? But only if we have his skill and control... we're so fucked.

Human psykers (with occasional exceptions, like Space Marine Librarians) tend to be one-trick ponies; if they are telepathic, then that is what they can do - it is very unlikely that they will have any skill in something like telekinesis, pyromancy, or divination. If this was an RPG, then they would be the guys who put all their experience into learning one or two skills until they have maxed them out, but end up having nothing left to spend on anything else. Yes, they can create a very powerful fireball, but that's all they can do. If anyone is capable of "omnimancy" then they are usually so powerful they can brute force powers from all disciplines, or are engaging in rituals and sorceries - which is technically cheating, and stupidly dangerous to even masters of the art. That said, human psyker squads form a sort of network of power through which they can do stuff far beyond what any one of them is capable of. So, a species-wide version of this would be pretty lit (or the easiest conquest Chaos has ever had). On the bright side, the Empyric Engineers will probably be allowed back into the Imperium and prove incredibly useful. There is also biomancy to consider. Imagine the Imperium being able to use biomancy to uplift the human body beyond biological limitations? But, their fanatical obsession with the divine human form preventing them from doing anything evil with it. Uh. By Imperial standards.

Squats/Kin

The Squats that grow very old are called "living ancestors". They are extremely rare, and develop psychic powers by communing with the spirits of their predecessors. And you could put them in an egg-shaped terminator armor, then onto a trike, with a bolt-firing cavalry lan-*BLAM* Squats are heresy! *FFSSSHHHHHOOOOOOOOOMMMMMM!* Oi! What did ye just call us? Uh... Nothing! Nothing at all!

Due to their ancestors' genetic modifications to protect themselves and their toys from being daemon chow, most Squats have souls nearly as dim as the Tau, and thus, little to no Psyker potential. However, as the Votann's data banks can only be accessed by Psykers, they created specialized cloneskeins that allow the Kin born from them to tap into the Immaterium in a more controlled and safer manner. These psykers are known as the Grimnyr and act as the Votann's eyes and mouths. They also have a thing for cosplaying wizards.

Ogryns

The Ogryn were initially thought by the Imperium to be too stupid to develop psychic powers, but were proved wrong sometime after the Great Rift by Cassia, who demonstrated psychic powers in battle by saving a Commissar. She ended up being recruited by Inquisitor Falx, and displayed other unique effects that were considered impossible to do by Ogryn: namely, being able to read and write on a basic level. Falx also believes (with a fair amount of supporting evidence) that other Ogryn are beginning to pop up with similar abilities, likely a result of the Rift itself. Fortunately for the Ogryn, their legendary devotion to the Emperor and love for their fellow, normal humans will probably be a major benefit to them in this situation. In several ways.

Forces of Chaos

Chaos tends to have...varied opinions on psykers.

• Daemons are made out of the stuff of the Warp, and using psychic powers is shaping and throwing out a little bit of the Warp, so a daemon using psychic powers is the rough equivalent of taking a severed limb or some bones and throwing it as a viable attack. While that sounds pretty fucking metal, Khorne thinks that pulling this bullshit is a sign that you're not manly enough to get into the thick of it and handle the situation on your own, an opinion shared by his daemons and mortal followers. Not only do they not use psychic powers, but they actively hate any pussified deadbeat who uses them, and even get toys that fuck with anybody trying to use those powers. However, it should be said that (deeply confused) Khornate traitor librarians exist - Khorne's opinion of them doesn't stop them from idolizing him. They just don't receive his favor, obviously (Unless your name is SSSSSSIIIINDDDRRRIIII). Any psyker who wishes to serve him swears off their powers except for utilitarian uses like telepathy or stopping other psykers from cheating. Having a cheat code and not using it makes you more honorable, not less.
• On the other end of the spectrum, Tzeentch, being the god of magic, is hugely into psychic powers, and he likes making new ones that do all kinds of mix-and-match effects, though only the best are actually rolled out. That's the official line. He's actually a closeted pyro, and a lot of his powers are burny or... do weird shit. Being the patron of psykers, if people get Tzeentch's attention because they're scholars or David Xanatos, they'll eventually develop psychic power whether they like it or not. Due to higher exposure to Warp energy via psychic powers and serving the god of change, Tzeentch's minions, and more often, their minions have the highest rates of mutation and turning into Chao--er, tentacly gribblies. Ahriman, possibly the greatest of Tzeentch's servants barring Magnus himself, was so powerful a Psyker that he was able to defeat Ulthwe's most venerated Psyker, Kyaduras the Anchorite, with ease, and, in the same battle, also casually defeated Yvraine, the Yncarne, and the Visarch.
• In the middle, Nurgle likes any psykers that spread his gifts around, but doesn't really go out of his way to promote them in his followers either, probably on account of his direct opposition to Tzeentch. Needless to say, his powers all revolve around inflicting people with horrible diseases or boosting their endurance to unnatural levels.
• Similarly, Slaanesh thinks Tzeentch is a giant nerd, whose overdevotion to his nerdy hobby of inventing new psychic powers will never get him laid. We think it says this because Tzeentch refuses to sleep with it (it almost certainly likes his tentacles and crotch mouth) is afraid he might catch Warp chlamydia, and he's just not willing to give Nurgle that kind of satisfaction. Slaaneshi psykers emphasize powers that make the hurt feel good, or have effects that look like the target might have snorted the psychic power off of a stripper's ass.

Being able to combine psychic powers, sorcery, and the favours of the Dark Gods easily makes the psychic followers of Chaos amongst the most powerful and dangerous individuals around. Such power comes at a price, of course. A Chaos corrupted or worshipping Psyker is significantly more to blow up into Daemons, both because they deal with them directly and thus increase the natural risk and/or because Tzeentch felt like a dick at the wrong time.

Eldar

Craftworld

The Craftworld Eldar are a strongly psychically-attuned race. While the regular, off-the-street Eldar will restrain themselves from using any psychic powers beside basic telepathic communication and the ability to use their psychic technology out of fear of being mindraped by Slaaneshi daemons, they can join the Path of the Seer and train to better control their innate abilities. Eldar who go on this path with previous experience as Aspect Warriors become Warlocks, while the most powerful seers can become Farseers and leaders of their people. They're ridiculously powerful psykers because of that deep imprint in the Warp to draw power, combined with their Spehss Ehlvan minds and willpower. They just need to be fucking careful and avoid unwanted attention unless they like tentacles. By "ridiculously powerful", the meaning is more "can do anything" rather than raw power. Humans in the setting have the raw power, Eldar have the flexibility. Like in almost everything else involving humans and Eldar.

The true capacity and depth of Eldar psychic abilities tend to be rather watered down or just ignored outright, due to the demands of the plot which is almost always told with the Imperium as the protagonist of the story. Within most stories the authors will tend to use the "M. Night Shyamalan" style of writing, which means having the Eldar Farseers getting a vision of events that are going to happen in the future and set out to stop it, only to discover that they either had it wrong the whole time, or they themselves brought about the very event that they wished to prevent; all the while the authors are hopping around shouting "WHAT A TWIST" at the top of their lungs. Due to the vast focus put upon the forces of the Imperium it is not a surprise that there is such an abundance of named human Psykers running rampant across the galaxy, but remember that humans only produce an exceptional psychic individual once in a blue moon; those humans that prove themselves strong and stable enough to join the ranks of the Marines as Librarians are a rare breed but more than prove their worth over multiple centuries of service. Librarians are also really damn powerful by any standard.

By comparison, every Eldar citizen can, if they wish to, potentially become a Farseer, though no Farseer will ever manage to reach the same level of power as the likes of Magnus (it's not really a fair comparison as he really isn't human and did not come into existence through natural means). Or a Warlock, which are the Eldar battle psykers. A Librarian will usually be more powerful as well, though not flexible in capabilities. In a slightly darker turn of events, where the likes of Spiritseers, who are able to commune with and manipulate the souls of the Eldar dead, the Ynnari actually amplify their existing powers through absorbing the souls of previous Eldar, which many Eldar among the Craftworlds consider to be horrifying. GW's "fuck what the Xenos are supposed to be able to do; the Imperium and Chaos are the stars of this setting" rule is seen many times in the story itself, with powerful Eldar Psykers like Eldrad very rarely performing impressive feats within the stories they are featured in, and you are just told that they are very powerful before they are inevitably defeated by the glorious human protagonist (Loyalist or otherwise). Eldar Farseers, in particular, suffer from the "Worf effect" more then most, despite often being heralded as great seers they have a terrible track record within the storyline itself, very rarely succeeding in any of their endeavors. Something Eldar tend to forget is that just because you foresaw something one moment doesn't mean the next moment it's still true. The future constantly changes and while the Eldar like to talk about that, that doesn't stop them from acting on the assumption that the futures they see are set in stone (and then being proven wrong). An important thing to keep in mind is that The Black Library writers love to add their very own super duper special human snowflake Psyker into the story because of the "Rule of Cool", with many in the lore being attributed with the ability to destroy Titans and armies. Farseers on the other hand are often shown being incapable of even defeating a single Space Marine let alone a tank, army or Titan, and Farseers are explicitly meant to be the most powerful Eldar Psykers in the galaxy. However, this could simply be either a way of keeping it so they don't win by default due to knowing exactly how a battle will go or (much more likely) Black Library incompetence making it impossible to get a remotely accurate depiction of what their powers can do. Or the more obvious possibility: Eldar Paths are extremely focused and a Farseer is not a battle psyker. Even Eldar Warlocks (their battle psykers) are more akin to Primaris Psykers than the likes of Librarians.

The 8th edition codex seems to have made an attempt to address this situation. Whilst all Eldar walking upon the Witch Paths are capable of mastering many different psychic disciplines throughout their long lives, they tend to avoid developing their more destructive powers, instead focusing all their efforts on developing their more “helpful” and “creative” powers; for example, the Bonesingers use their powers to create and then craft Wraithbone. This goes well with the Craftworlder’s mindset when it comes to avoiding their darker, more destructive sides.

Those who find it easier to master the more destructive disciplines tend to be those who have walked the Path of the Warrior and have already had their more violent impulses tested. These individuals will become Warlocks, and can make use of their more destructive powers upon the battlefield without putting those around them in danger as they already have their War Masks in place, to help focus them and to separate their warlike selves from their normal selves.

Farseers will unfortunately still suck in any story that they are featured in, but they are not supposed to be just weapons, they are guides and advisers who help guide the blade, not swing it themselves. This, of course, doesn’t help the fact that they are still going to be treated like a bunch of redshirts that will inevitably fail at every single opportunity by the writers at GW.

The Eldar make use of Rune Stones that act as conduits through which they can manipulate the psychic energies of a the Warp without actually having to interact with the Warp directly; though this does limit their overall psychic power level it provides them with a much safer and more controlled way to access the energies of the Warp. The Rune stones also act as psychic fuses, burning out and cutting the link to the Warp should an Eldar try to tap into too much power before they are ready. As they develop their control and skill the Eldar psykers will eventually be able to use a far greater number of stones, which allows them to tap into far greater pools of power.

Eldar tend to be the Swiss Army knife of psykers; being able to learn many different psychic disciplines. If this was an RPG then they would be the guys who spread their experience out over a wide range of powers and abilities. They don't tend to max out their powers, but instead end up with a much wider skill set to call upon.

Harlequins

Harlequins are also psychic, which really comes out in their shows; although only the Shadowseer can use powers in a fight, other Harlequins enjoy the benefits of psychic senses and silent telepathic communication.

Dark Eldar

The Dark Eldar, on the other hand, are psychically atrophied compared to their Craftworld bros due to centuries of largely abstaining from using their powers. They have to avoid using psychic powers due to Slaanesh's "I own your soul" gig, their general lack of restraint (required for psykers to use their powers without instantly getting devoured alive by daemons), and the reasonable feared of becoming bait for snusnu with a Daemonette.

They can use psychic artifacts like all those crazy living swords and Haemonculus toys, and the only psychic power they use is their infamous emotion-vampirism ability that can replenish their bodies and souls by inflicting copious amounts of pain and misery and soaking in all that torment. And kamehamehas, for Mandrakes. They do have extra fun torturing, killing and then weaponizing psykers though.

Exodite

While the psychic capabilities of Exodite Eldar are rarely elaborated upon (mostly because works that involve them typically have them being punching bags for enemy factions), it is likely that the Exodites still retain their psychic powers, as they are able to communicate with their animal allies (beasts of burden and war mounts) and influence the planet they're on into being living weapons by manipulating the flora.

Both of these would fall under the categories of telepathy and biomancy, respectively.

Tau Empire

The Tau race is incapable of producing psykers due to having almost no presence in the Warp. The good news for them is that they don't have to worry about Bad Things, such as any members of their species spontaneously turning inside out from the anus and summoning an army of monsters that will fuck, kill, and eat people(not strictly in that order). The bad news is that they have no way to natively develop psychically-based technology, so their FTL travel is glacial (albeit a good deal safer than an actual Warp jump) and their FTL communications options are limited to the Space Pony Express. It should be noted that some of their vassal races have psychic abilities, including their own Navigator analogues. It is known that they actually locate, isolate and study human (and probably other species') psykers, but as the Immaterium is too irrational and far beyond their science they have had little success in analyzing the reason behind Gue'La suddenly going crazy and summoning teleporting armies of homicidal hellspawn aliens out of thin air. Of course, following what happened to the Fourth Sphere Expansion, they now have little reason to stay ignorant of those things that come from nowhere...

• Funnily, the Kroot have psykers (they have a diverse diet) and understand a great deal about the Immaterium. They even have some crude warp vessels, usually with purchased or stolen xeno tech in it.
• The Nicassar are another auxilia race that just happens to have psykers of their own, though their potency when compared to the big boys of the setting hasn't really been examined. Although for some reason BFG lore also established that they don’t even use FTL, they just piggyback on Tau ships. Before they met the Tau they would go into hibernation and travel at slower-than-light speeds between planets.

Necrons

The Necrons no longer have souls, so they have no presence in the Warp, and therefore can't manipulate it with their minds. And yet they have managed to conquer the Warp with technology. (Egyptian Skeleton Zombie Robots, Yeah!) They keep fucking up the Warp and psykers with the gloom prisms they mount on their spiders, pylons (sounds familiar...), and even giant city-sized mechanisms that cut entire planets from the Warp. One in the Word Bearer novels even had this... thing, about the size of a resurrection orb, which blanketed an entire system in a "psychic black hole." Problem, daemons?

Before the 5e fluff changes, they also had Pariahs, specialized soldiers made from Blanks that acted like Culexus assassins.

Orks

The Orks are all passive psykers, or rather emanate their own gestalt "psykik" energy in the power of the WAAAGH!, though they're mostly unaware of this. A large number of Orks in one place will slightly alter reality to suit their wishes, turning jam-prone, dirty blunderbusses into fully-automatic rifles of rape. Weirdboyz are the only ones who can tap into it actively to manifest psychic powers, which have an amusing tendency to make Orks heads explode with absolutely no warning. As said above, the Orks draw psychic energy from each other, not from the Warp, which makes them virtually immune to Chaotic influences(but not completely immune), although they do have a pretty serious presence in the Warp in the form of Gork and Mork (or maybe the other way round).

Tyranids

The Tyranids are innate psykers and their collective power comes from the Hive Mind, which is what the big bugs tap into to manifest psychic powers rather than the Warp, the same way a Weirdboy would tap into the WAAAGH!. Only a few bugs are capable of weaponizing the Hive Mind's psychic energy, the Zoanthrope being one such option for Tyranid psychic support. Few 'Nid species beyond the Zoanthrope are capable of firing tank piercing mind-lasers; instead the other big bugs use their powers for buffs and a form of control called "synapse". Synapse is essentially the presence of the Tyranid Hive Mind being made manifest, allowing it to control and direct the lesser non-synaptic Tyranid creatures, as the non-synaptic creatures are essentially just soulless animals without any overarching will of their own beside their own instincts. In the presence of a synapse creature, such a being is then "ensouled" with the Hive Mind who acts through that creature's body. If the Hive Mind gets driven away from the horde through psychic shenanigans or the synapse bug gets killed, the synapse link collapses and the 'Nids turn feral, becoming essentially really dangerous wildlife. Before Genestealers got retconned into being Tyranids, Genestealer Magi did use the power of the Warp to power their abilities and could dedicate themselves to Chaos. There's an image of a Khornate Magus in one of the Rogue Trader Chaos books, though that is nonsensical in itself given Khorne's views on psykers.

Psychic Power Hierarchy

The Imperium of Man uses a 24-point scale (with gradations named for the letters of the Greek alphabet, for some reason; there are 26 points counting the "top" and "bottom", both of which are used to classify entities beyond the scale's ability to measure) called The Assignment to determine the comparative psychic power of an individual. Unfortunately, this gets very confusing in the fluff, where Alpha-level psykers seem to be a dime a dozen and show up all over the place, and established characters that are canonically high on the scale are completely overshadowed by some other character. All of this "logically" leads neckbeards to reach the flawed conclusion that their favourite character on the tabletop MUST be Alpha level or greater.

Furthermore, it is canon that a psyker's place on the scale can change over time as his or her powers either increase with practice or decrease due to atrophy from lack of use. So generally, the fluff is quite useless for determining who is greater than whom; without the evidence of a direct psychic battle between any two to compare how they match up, it is pretty pointless to use this method.

Lastly, this is an Imperial system, and likely should be used only to assess humans. While chances are someone in-universe has looked at a wyrdboy and gone "Theta-ish", you shouldn't since it's comparable to using metric tools on imperial fasteners: annoying, inappropriate, and probably a crime according to both the Mechanicus and Administratum. So don't do it.

Positive Psychic Levels

1. Alpha-Plus
   • Alpha-Plus psykers do not represent the top of the scale, but are completely beyond the scale. In fact, the scale theoretically continues upwards beyond Alpha-Plus into truly godlike tiers of power, although such individuals have never been encountered, barring maybe the Emperor. Even so, Alpha-Plus individuals operate on a completely different wavelength from normal people and are owners of completely alien mindsets. In terms of power, they are akin to weapons of mass destruction, as in theory there is nothing that a trained Alpha-Plus psyker cannot accomplish through force of will. There is a tacit assumption that this category contains the Emperor and his "notably-psychic" Primarchs (especially Magnus the Red in his daemon Primarch form), but even he admitted there were limits on his power, specifically that one could not be both all-seeing and all-powerful at the same time. No attempt is made to distinguish power levels among members of this grade, because they're all so absurdly strong there's no point in quantifying it. To put it in context, an emergent Alpha Plus psyker - one not fully in control of their abilities - was able to destroy a starship, freeze time, put it back together and resurrect its crew from the dead before removing themselves permanently from the physical universe in an act of unparalleled self-control and incredibly heroic self-sacrifice to defend humanity from that psyker’s risk of possession. To dumb it down for the rest of you idiots, that the same as snapping an Imperator Class Titan in half with a sneeze at HALF A SECTOR AWAY!
2. Alpha / Beta
   • Exceedingly rare and dangerous. The Imperium believes that human beings have not sufficiently evolved enough to contain Beta and Alpha levels of psionic talent without going completely batshit mental, though exceptions do exist. Arvann Stern is confirmed to be Beta-level, and he's not even the most powerful psyker in his chapter. Beta seems to largely be the limit of human sanity, though this is probably fixed with heavy amounts of cybernetics and hypno-indoctrination to make the crazies into useful, if mindless, tools. Most psykers of this power level are considered too dangerous to control and are likely to be sacrificed to power the Astronomican, while very unlucky Alpha psykers may instead find Imperial service in the Ordo Sinister, meaning they'll get turned into screaming batteries for Psi-Titans. The unluckiest Alphas and Betas get to be Grey Knights, and fight daemons until their painful death in battle. The luckiest ones become specialized Astropaths whose predictions of the future are used by the Adeptus Custodes to find threats to Terra.
3. Gamma / Delta
   • Approximately one out of every billion human births will produce a Delta or Gamma level psyker, who, for the Imperium, represent the "useful" upper limit of psychic potential without being too dangerous to let live, though it's up to the Inquisition to be the ultimate judge of that. Most psykers of this power are either slain or possessed if they are not discovered quickly by the Adeptus Astra Telepathica. The Inquisitor Gideon Ravenor was believed to be a high Delta-level psyker.
4. Epsilon / Zeta
   • Epsilons and Zetas represent relatively powerful psykers, who are strong enough to be considered threats to security by the Imperium.
5. Eta / Theta / Iota
   • Iota represents the first "true" psykers who are able to manifest and control psychic abilities with a modicum of training. Anything below this point is not considered a "true" Psyker.
6. Kappa / Lambda / Mu / Nu / Xi / Omicron
   • People who can manifest psychic talents either unconsciously or so subtly as to be beneath the notice of everyone but the Inquisition, such as performing minor "magic" tricks, divinations, or other weird occurrences generally dismissed as good luck. Most of the time they are quite useless, but still represent a danger since they can be possessed. Although the Inquisition is too busy to sort out everyone at this level, the Imperial public is conditioned to spot and ostracize these witches if the truth about them ever comes to light in their societies. Those that do get picked up by the Black Ships are generally considered not worth the risk of training and almost always receive a one-way trip to the Golden Throne to have some sort of use wrung out of them. If you’re power level is low enough, though, even the Inquisition and Black Ships will ignore you as a non-threat. Which sounds like a damn stupid thing for them to do, really.
7. Pi / Rho
   • Run-of-the-mill humans, neither psy-active nor psy-inert, but indisputably on the positive side because the negatives... get weird.

Negative Psychic Levels

1. Sigma/Tau
   • When you start descending below the scale of humans, into the "negative" side of the scale, you start going psy-inert. People at the Sigma/Tau level would be able to perceive and be injured by an incoming fireball but may not be able to discern that it came from an unnatural source. It is believed the entire Tau species rests at this level.
2. Upsilon / Phi / Chi / Psi (ironically)
   • These are individuals with varying degrees of immunity to psychic powers, though not to the same extent as a true Blank or Pariah.
3. Omega
   • Usually referred to as Untouchables, Pariahs, or Blanks. Not only are they immune to psychic powers, but they can also inhibit the powers of psykers around them and/or neutralize psychic effects on an area around them. The Imperium often employs Blanks as an Inqusitor's servants, while the Sisters of Silence are a trained order of Psyker-huntresses that are all Blanks. The Eldar, believe it or not, also produce the occasional Blank, and these become the infamous Solitaires of the Harlequins. True Blanks have such a psy-negative effect that even normal people can sense it, and it manifests as an irrational loathing for the Blank, though in some cases it can be offset by either a good personality (Alizebeth Bequin) or an odd, but explainable character trait (Gunner Jurgen). For psykers however, Omega-level persons are physically and mentally painful to be near.
   • The fluff is a bit inconsistent on the exact limit of a Blank's ability, but that's fine, because in Dark Heresy, Blanks exhibit a broad range of effects which can be different from person to person, much like high-level psykers can have different powers and disciplines. It can also be trained and manipulated, as in the case of the Sisters of Silence. Others can train themselves to actually benefit from the psychic power they suck from their surroundings (having psykers or even normal living beings in those surroundings really helps), which they use to empower and invigorate themselves.
   • There have, however, been a few occasions where a Blank's abilities have been overwhelmed or even permanently shutdown. Bequin's blankness was overwhelmed by the warp corrupted psi-sentience of a Chaos Titan and Wystan Frauka's blankness was gradually eroded away over time until he was able to be possessed by a psyker. In Gathering Storm, when Khaine's Gate broke, all the strapped and likely sodomized Blanks that were chained to it and had been screaming for the past few decades either exploded into ludicrous gibs or got outright possessed. Alpha Plus Psykers like the Emperor can probably brute force their way through even the most unnaturally powerful blanks.
4. Omega-Minus
   • Strictly limited to Culexus Assassins, who are given specialized equipment to maximize the effectiveness of their Blank abilities. Another example was an Omega Minus Embryo kept in a stasis jar that was so potent that it killed a Thousand Son's Sorcerer and an Eldar Harlequin just by being near them.
5. ???
   • The Pariah Alpha from Inqusitor: Martyr is a bit of a wild card, since we don't know if there are more like her or what she's capable of. We do know at least their mere presence can truedeath Daemons.

Other Factors

When you get further into the details, not only is sheer psychic potential a factor to consider in determining how powerful a psyker is, there are other things to consider when trying to figure out mechanically how one differs from another.

This is easier to do outside of the tabletop setting which focuses on a d6 scale, where a minor difference in a single statistic can create one power tier an order of magnitude more effective above another.

Level of Control

The Willpower, discipline and training (read: control) of a given individual holds about as much sway as psy-rating in saying who is more powerful than who. Probably because if you can’t use what you have then the excess doesn’t count, and probably also because the Warp responds to will and so willpower can easily become literal power. This is most clearly represented in the RPG rules, but often shows up in the fluff, particularly in instances where powerful but neophyte psykers are only coming to learn their powers (such as Da'Kir of the Salamanders or Zahariel of the First Legion) A lot of commentators forget this and focus purely on psy-rating when attempting to compare psychic ability. A psyker may have a high psy-rating but if their ability to control their powers are low then they are less likely to successfully manifest but are more likely to create a powerful effect when they do; this is in fact part of the reason why the Imperium considers them to be so dangerous. Inversely, a low-mid rated psyker with a strong grasp on his abilities is more likely to successfully manifest and is also more likely to be able to resist/counter incoming psychic effects.

Interestingly enough, even non-psykers with enough discipline and mental fortitude can be trained to resist psychic powers, though this does not mean they are necessarily psychically blunted themselves. Typically this ability comes in the form of intense indoctrination or rote training, or from sheer force of belief and faith to literally "Deny the Witch". It’s unsurprising given the nature of the Warp.

Sorcery

The second thing to consider is the whims of the Chaos Gods, (particularly Tzeentch). Sorcery is entirely separate from Psy Rating/Assignment and even a non-psyker with the correct rites and incantations can pull off similar (but not the same) manifestations to a true psyker. Back during the Horus Heresy, Erebus (and 30k Word Bearers with Burning Lore) was an example of this. Furthermore, although Khorne himself hates psykers and sorcery, the Daemonancers among his own followers do have access to rituals for summoning daemons; otherwise they would be solely reliant upon tears in the warp for receiving daemonic assistance. These ritualistic functions may have been carried over to 8th edition where every chaos character now has the ability to summon daemons (though this could just as plausibly be the champion calling out to his God and being rewarded with assistance while in a location where the realspace/warp barriers are thin or have already been broken)

A true psyker with access to sorcery can increase his power levels even higher and can easily outstrip more powerful psykers than himself, though Sorcery comes with its own perils and generally leads to Damnation and Corruption (no wonder big Emps put his foot down at the council of Nikaea) not to mention the idiocy of increasing your psychic power beyond what you can control.

Furthermore, certain favours gifted from the Gods also increase psychic output. It used to be that in days gone by, the Mark of Tzeentch used to enhance psychic potential on the tabletop. Nowadays it doesn't, but in the RPG it still does. But obviously once you've been marked by a God of Chaos, you're in their pocket forever, and you best do your utmost to keep them happy or you end up becoming that-which-shall-not-be-named.

Pushing

Separate from either issue is the fact that Psykers can theoretically increase their assignment rating for a brief instant, drawing surplus energies from the warp and "Pushing" their powers even further than should be possible, basically taking a gamble and temporarily adding MOAR power to their manifestation.

In both the rules and the fluff there are common instances of psykers being able to boost themselves and create larger effects or to overcome more powerful opponents. This was partially illustrated with the 7th Edition ruleset and the shared Warp Charge "Pool", where low level psykers could throw all available warp charges into a single power and overcome the abilities of a higher mastery opponent, albeit at considerable risk of Perils of the Warp.

However, not all psykers have the ability to do this to the same degree, and although not adequately shown on the tabletop such rules were presented in the RPG settings, which broadly fit the established fluff:

• Bound Psykers, that is to say, nearly every Imperial psyker who has undergone a Sanctioning process [Including Librarians] or Soul-Binding can only push themselves so much and they are at greater risk of something bad happening (ie Perils of the Warp) when they do so. Specifically, Soul-Binding to the Emperor includes all Astropaths and the psykers who lack the willpower to be safely trained but have too much power to be waste.
• Unbound Psykers (Rogue Psykers / Sorcerers) on the other hand can push to even higher degrees of power but shit WILL hit the fan and can almost guarantee their manifested power will come with some form of unintended side effect when they do.
• Daemonic Psykers who attempt to push their powers can go yet higher, but are considerably more likely to cause dangerous psychic phenomena. However since their mere existence on the material plane can be considered a dangerous psychic phenomena they are less likely to be bothered by it unless it is a straight up Perils of the Warp.

Psychic Disciplines

See its page.

Gallery

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  You should never invite psykers to your poker night.

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  Culexus assassins are psykers' worst nightmare, though they are very friendly.

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  Psyker Dredd knows you were gonna say that.

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  Shh, my heresy sense is tingling...

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  The Adeptus Astra Telepathica's easy steps in identifying and capturing potential psykers.

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