“We always vilify what we don't understand.”
- Nenia Campbell

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 raped in countless ways by Daemons and used as a gateway from the Warp into the physical world. Luckily, when 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.

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 cool. There is also the threat of Chaos possessing them and summoning daemons. 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, serve to 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 has; 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; but on most worlds being sactioned (assuming you survive and don't mind the side affects) 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. 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. People still feel uneasy around them, but even Black Templars have to cope with them and make nice faces. Pointing out that the Emperor was a psyker usually results in the person struggling to make a retort for a few moments before beating you to death. Well, actually not, since for an average citizen of the Imperium the Emperor is God, so Him 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.
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. So it's an opinion shared by his daemons and his mortal followers. Not only do they not use psychic powers, but they actively hate any pussified deadbeat who uses them, and they even get toys that fucks with anybody trying to use those powers.However, it should be said that Khornate traitor librarians exists, labrarians by nature being beings that focus on destruction with their minds so Khorne recognizes that as inner strength, turning a blind eye on them
- 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.
- In the middle, Nurgle says he hates psychic powers, but only because he really hates Tzeentch. The reason is believed to be because Tzeentch got The Changeling to whoopie cushion Nurgle, and Papa N is mad that ~~he hadn't thought of that first~~ it wasn't "the Plague Wind" going out to infect an Imperial orphanage or something. Papa Nurgle, being the affable old fart that he is (*snicker*) actually likes any psykers that spread his gifts around. 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), we think because he's afraid he might catch Warp chlamydia, and he's just not willing to give Nurgle that kind of satisfaction. Slaaneshi psykers emphasize powers that makes the hurt feel good, or has effects that look like the target might have snorted the psychic power off of a stripper's ass.
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 on this path are the Warlocks, the most powerful of which 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 never use their full strength unless they like tentacles.
- Harlequins are also psychic, which really comes out in their shows, and although only the Shadowseer can use powers in a fight, other harlequins do enjoy the benefits of psychic senses and silent telepathic communication.
The Dark Eldar however, are psychically atrophied compared to their Craftworld bros. They have to avoid using psychic powers due to Slaanesh's "I own your soul" gig, and DEldar using psychic powers are reasonably feared as bait for snusnu-with-a-Daemonette. They can use psychic artifacts, like all those crazy living swords and haemonculus toys; the only psychic power their infamous emotion-vampirism ability that can replenish their bodies and souls. And kamehamehas, for mandrakes. They do have extra fun torturing, killing and then weaponizing psykers though.
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 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 armies of violent omnicidal "aliens" out of thin air.
The Necrons no longer have souls, so they have no presence in the warp, so they can't manipulate it with their minds. And yet, they have managed to conquer the warp with technology. (Russian Robot Skeleton Zombie Robots, Fuck Yeah!) They keep fucking up the warp and psykers with the gloom prisms they mount on their spiders, pylons (sounds familiar...), or 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 caused blanketed an entire system in a "psychic black hole." Problem, daemons?
The Orks are all passive psykers, or rather emanate their own gestalt "psykik" energy, in the power of the WAAAGH!. 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. Wyrdboys are the only ones who can tap into it actively to manifest psychic powers which 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, although they do have a pretty serious presence in the warp in the form of Gork and Mork (or maybe the other way round).
The Tyranids are innate psykers, and their collective power goes into the Hive Mind, which is what the big bugs tap into to manifest psychic powers rather than the warp, the same way a Wyrdboy would tap into the WAAAGH!. Only a few bugs are capable weaponizing the Hive Mind's psychic energy, the Zoanthrope being famous for Tyranid designated psychic support. Few Nid species beyond the Zoanthrope are capable of firing tank piercing mind-lasers, instead the other big bugs use it powers for buffs and a form of control called "synapse", which keeps the Tyranid horde from going feral and killing everyone and everything they see. If the Hive Mind gets driven away from the horde or the synapse bug gets killed, the synapse link collapses and the nids turn feral. How Zoanthroapes still manage to remain psychic despite losing their link the Hive Mind is still not understood either. Also, 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 Khorne Magus in one of the 2ed books...
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!

Psychic Power Hierarchy

The Imperium of Man uses a twenty-four point scale to determine the comparative psychic power of an individual, unfortunately this gets very confusing in the fluff, especially where Alpha-level psykers seem to be a dime-a-dozen and show up all over the place... Or when established characters are canonically given a very high rating (Brother-Captain Stern = Beta / Ravenor = Delta [retconned back from Alpha-Plus in the fluff]), but then are completely over shadowed 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.

Not only that, it is canon that psychic assignments can change over time as powers increase with practice or atrophy due to lack of use. So generally, the fluff is quite useless for determining who is greater than whom; without evidence of a direct psychic battle between the two comparing how they match up to each other is pretty pointless using this method.

Nevertheless, here is the scale:

Alpha-Plus

Alpha-plus does not represent the top point of the scale, but in fact are completely beyond the scale entirely, these 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 (depending on their particular discipline; for instance a Alpha-Plus Pyromancer is entirely different from a Telepath) However, in theory there is nothing that a trained Alpha-Plus psyker cannot accomplish through force of will; from snapping a Titan in half to summoning a legion of Greater Daemons.
There is the tacit assumption that this category contains the Emperor and his "notably-psychic" Primarchs (unlike Corax & Sanguinius who may or may not have been psychic at all) 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.

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 bat-shit mental. (though exceptions do exist)
Canon Imperial Alpha level psykers who still have their mental faculties happen to be Grey Knights (Epimetheus & Hyperion [Stern is listed as Beta])
It is very much assumed that the more powerful fluff characters, such as Eldrad or Ahriman are Alpha-Level

Gamma // Delta

Approximately every 1 per 1,000,000,000 human births you get a Delta / Gamma level psyker who 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 judge of that.

Epsilon // Zeta

These represent your ranks of relatively "powerful" psyker.

Eta // Theta // Iota

Iota represents the first "true" psykers, and are able to manifest and control psychic abilities with a modicum of training. These are the people that find their way into the various Imperial institutions.

Kappa // Lambda // Mu // Nu // Xi // Omicron

You get a broad range of 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 or divinations. Most of the time they are quite useless but still represent a danger since they can be possessed.
Despite the Inquisition being too busy to sort out everyone at this level, the Imperial public is conditioned well enough to spot and ostracize these witches if the truth ever came to light. (read Gaunt's Ghosts)

Pi // Rho

Basic Humans, neither psy-active or psy-inert. A generally safe place to be.

Sigma // Tau

When you start descending below the scale of humans you start going psy-inert. At the earliest point, when psychic effects start manifesting subtle phenomena that everyone describes as "something feeling wrong" these people can't even tell anything is happening.

Upsilon // Phi // Chi // Psi

Subjects with varying degrees of immunity to psyker powers, not true blanks, but quite possibly enough to reduce an incoming power from absolutely lethal to something survivable or are much more difficult to mind-read than normal.

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 neutralise effects on an area around themselves. The fluff is a bit inconsistent on the exact limit of this ability, but that's fine, as in Dark Heresy Untouchables represent a broad range of effects and can be different from person-to-person, much like high level psykers can have different powers and disciplines.
True Untouchables have such a psy-negative effect that even normal people can sense it, and manifests in an irrational loathing for that person, though in some cases it can be passed off by either a good personality (Alizebeth Bequin) or an odd character trait (Gunner Jurgen). For psykers, however, Omega level persons are physically and mentally painful to be near.
After some training Pariahs can actually benefit from the psychic power they suck from their surroundings (having psykers or even normal living beings in that surroundings really helps), which they use to empower and invigorate themselves, and Culexus assassins move it even further with their Animus Oculum helm by shooting mind-bullets, formed from that energy.

Omega-minus

Yin to Alpha-plus Yang, Black Pariahs are so extremely rare it's a safe bet only one of them ever existed in our Galaxy at any point of time. In fact there are only two persons of that level in the fluff (Spear, and a festus inside the Black Lanterns). Their null-aura is so powerful it drives even normal humans insane (and eventually kills them, unless they kill themselves first), and psykers, daemons and psychic artifacts within their aura simply cease to exist, turning into dust in a matter seconds. Even other Pariahs are terrified of them. Basically anybody with the classification Omega-minus is a warp black hole that only exists to suck in all souls and chaos energies and turn it into void. That means the people who get killed by these fucking terrifying abominations has their soul sucked out and completely destroyed. It's not as terrifying as it sounds, though, as being utterly destroyed upon death certainly beats being raped by daemons for all eternity.

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 focusses 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 (read: control) of a given individual holds almost about as much sway in saying who is more powerful than who, so just because a psyker has an mid-high psy rating, if their Willpower is low (say: 30) they're less likely to successfully manifest but are more likely to create a powerful effect when they do. Inversely, a low-mid rated psyker with a strong willpower (say: 60) is more likely to successfully manifest and is also more likely to be able to resist/counter incoming psychic effects.

That said, even in the mechanics of the RPG rules, once a psy-rating reaches epic heights then they should have no problems casting even with a low willpower.

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 (if not the same) manifestations as a true psyker. Back during the Horus Heresy, Erebus (and 30k Word Bearers with Burning Lore) is an example of this.

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)

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 in order to grant themselves more power.

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 is not so prevalent on the 40k tabletops (although with the 7th Edition advent of the shared Warp Charge "Pool" this may be untrue) and not all psykers have the ability to do this to the same degree.

One method is to "Push" their powers further, basically taking a gamble and temporarily adding MOAR power to their manifestation.

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.
Unbound psykers (Rogue Psykers / Sorcerers) on the other hand can reach even higher degrees of power but shit WILL hit the fan.
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

There are five "basic" psychic disciplines in the grim darkness of the future. Generally speaking psykers prefer to focus on one discipline if they seek more power, but doing so runs the risk of changing their mindset, as connection between psyker's mind and the Warp is two way, and if one is to use his mind to alter the Warp, warp would use that connection to alter psyker's mind. Some psykers fight this, some don't, and most don't live long enough for this to take effect.

Biomancy is about flesh/life force manipulation and shooting lightning, somehow (depending on your interpretation, either pyromancy or telekinesis fits this power better). Old and powerful biomancers tend to become quite sick fucks, building cruel and sadistic tendencies over time.
Divination is about predicting and tweaking the future. Old and powerful diviners are known to be extremely nerdy, hoarding massive ammounts of knowledge about deciphering visions and omens and general theory of the Warp, but this often comes at the price of constant melancholia and depression, as they realize how much of a puppets in the hand of Fate they are. Unless that diviner is Ahriman, who decided to skullfuck Fate and be the one in charge of his own destiny (though even he went though an "I'm just a puppet" emo-phase before it).
Pyromancy is about conjuring and throwing fire at people. It comes to no surprise to anyone that pyromancers that happen to live long enough without blowing themselves up often to grow into aggressive pyromaniac hotheads that tend to solve all problems by KILLING IT WITH FIRE.
Telepathy is pretty much self-explanatory - reading and translating minds. Mind control and mind rape goes there, as well as invisibility, even though this power renders you invisible to things without minds. In terms of long-term personality changes Telepaths draw the absolutely shortest straw, as over time they "pick up" bits and pieces of personalities of people they mindraped, risking to lose their original selves eventually, end even if they have the will to resist it, they inevitably grow into absolute misanthropes due to knowing too well what disgusting things people around them think about all the time.
Telekinesis is also obvious - moving things with a mind. Here also goes psychic shields and somehow dimensional gates. Not much known about how it affects psyker's psyche - old telekines are known to being quite stoic, even stubborn but many unspecialized psykers that manged to survive for long without losing their minds also tend move towards this archetype, so it's debatable whether it's discipline effect or a simple natural selection.
Daemonology is a sixth common discipline, although it's by no means basic, being much more difficult to learn and dangerous to use (as anything that directly deals with chaos daemons carries the possibility of the psyker's head exploding into a warp portal). It's split into two sub-disciplines - Malefic is about summoning daemons and using their power, and Sanctic is about banishing them and killing things with the refined unshaped Warp energy. Generally speaking most dedicated daemonologists do not live long enough to experience any personality-changing effects of their discipline, and those that do are either brainwashed on a regular basis (Grey Knights) or are already Chaos-corrupted and affected by daemonic pacts (Chaos sorcerers and wild psykers).

Besides those six, other disciplines exist, although they are mostly faction-specific:

Chaos psykers and daemons use their own specific disciplines, dedicated to one of the three gods who doesn't hate sorcery:
- Tzeentch is mostly about mutation, subjugation, and manipulation of the warp itself. Tzeentchian powers typically revolve around controlling the energies of the warp itself to damage foes (like Doombolts), mind-reading and controlling people, and manipulating reality to either open warp rifts or mutate their opponents into those-which-shall-not-be-named.
- Nurgle is like a twisted version of Biomancy, focusing on the psyker spreading the gifts of Papa Nurgle by polluting their surrounding area with his many plagues and contagions, causing most people to succumb and die within minutes.
- Slaanesh tends to be the trickiest, focusing on illusions, and manipulating their targets' senses to either control them or drive them into the throes of insanity (Which can either be good or bad depending on the power involved).
Eldar use runes as proxies to manifest psychic powers, so they don't get mind fucked by Slaaneshi daemons the moment they surge in the warp for the power, and they created two disciplines to utilize those runes:
- Runes of Fate Used by Farseers is kind of like Telepathy and Divination mixed together and shaken a bit - same kind of future telling and mindrape in a slightly different form. Except for Eldritch Storm, which looks like it was pulled form the Sanctic Daemonology
- Runes of Battle Used by Warlocks and Spiritseers. Features a set of powers that can be used to buff allies or debuff opponents, most of which doesn't fit into any of the basic disciplines.
Harlequin Shadowseers also use their own discipline called Phantasmancy, which is basically Telepathy on acid trip, as the psyker channels his own killer-clown kind of batshit insanity into people, and polishes it out with hallucinogen grenades thrown on the top.
Eldar Corsairs, being cool space pirates do not utilize rune stones, and channel psychic powers the old way, at a higher risk of being raped by daemons. Their old-school techniques allow them to use pre-fall majick called Aetheromancy, designed to utilize the Webway (much similar to how Sanctic Daemonologists utilize the Warp), so they can project a guidance maps, teleport their allies through it and create rifts that suck their enemies into it.
Ork Weirdboyz have their own discipline, that run on Waaagh! energy rather than the Warp, and don't have access to other disciplines. Surprisingly, Waaagh! powers are quite powerful and some of them are tricky, although they are mostly focused on pumping the boyz around the Weirboy with buffs, getting them closer to enemies or killing shit with green lightning.
Similarily, Tyranids, whose psychic powers also run on a collective psychic field have a specific discipline, which is Telepathy with a bit of Biomancy and the odd Zoanthope mind bullet power thrown in.
Spaсe Marine librartians also devaloped not one but FOUR special siciplines:

Technomancy Which is a telepathy/biomancy for machine spirits rather than living beings, for those who prefer the company of machines or like to wreck enemy ones.
Fulmination Biomancy that throws away all that life force and biochemistry shit and focuses solely on the lightning. All the lightning!
Geokinesis If you ever wanted to be an earthbender this is the discipline for you. Among other things it allows you to move buildings, hills and forests around.
Librarius Very Eldar-runes-like telepacy/divination based discipline trickery thrown in.

Space Wolf Rune Priests use a discipline called Tempestus (although they refuse to admit it's a psychic discipline at all, since they're hypocrite assholes) - it involves powers that mostly revolve around ice, earth and storm. It looks like it utilizes a psychic ice effect, which is a common side effect of manifesting powerful non-pyromancy powers, and then they mix that with a bit of Telekinesis to spin that ice around or shape it into giant killer wolves that eat you alive.
Blood Angels and their successors claim to have their own discipline called Sanguine but it's really just a bunch of Biomancy, Telekinesis and Telepathy powers thrown together and given fancy names. Their buffing spells have a tendency to affect allies, rather than just the psyker at the cost of sheer power, possibly on the selfless nature of their chapter. And then there's the Blood Lance--which is just the entirety of their psychosis bottled into one power--all the heat and fury of the Black Rage with the color of the Red thirst
The Dark Angels utilize the Interromancy discipline, which is a specialized variant of the Telepathy discipline, originally designed to interrogate ~~Fallen~~ FOUL TRAITORS WHO ARE NOT IN ANY WAY CONNECTED TO THE FIRST LEGION. It may lack some of the most bullshit OP powers of true Telepathy, but has a strong combination of buffs and debuffs and the later have a chance to permanently cripple their targets, especially if they are weak-willed.

Comparing Psykers

Addendum: This list is relative in the extreme. Trying to compare the Psychic powers of so many characters is fukkin' stupid.

0. The Hive Mind
1. God-Emperor of Mankind
2. Magnus the Red
3. Eldrad; Malcador the Sigilite; Azhek Ahriman.
4. Lorgar; Epimetheus; Warlock Titans.
5. Eldar Farseers; other Alpha level psykers; Lords of Change.
6. Rubric Sorcerers; Tigurius; Sevrin Loth; Mephiston; Njal Stormcaller; Ezekiel; Kaldor Draigo.
7. Chaos Sorcerers; Grey Knight Librarians; Ravenor; Hyperion; Swarmlord; Zaraphiston; Shadowseers (Harlequins); Spiritseers; Worldsingers (Exodites); Wraithseers (power equal to the seer used in its construction).
8. Regular Librarians, Zoanthropes; Eldar Warlocks; Bonesingers; Corsair void dreamers (tend to be random in power with some being the equal of Farseers).
9. Primaris Psykers; Hive Tyrants;
10. Wyrdwane/Sanctioned Psykers; Ork Wyrdboyz.
11. Regular Grey Knights; Navigators; Astropaths (they are powerful, but their powers are normally so specialized on being a cell-phone, most of them are no match for 'true' psykers).
12. Regular Eldar, Bane of Sanity Marines.
13. Regular Orks.
14. Regular Humans.
15. Blunted Humans; Tau; Space Wolves
16. Necrons
99. Blanks/Pariahs
100. Culexus Assassins/sisters of silence - farthest from Psyker as possible, soulless anti-psykers.
1000. The Black Pariah

I'm not sure about the place of Greater Daemons here; also, Tyranid psykers.

Gallery

You should never invite psykers to your poker night.
Culexus assassins are psykers' worst nightmare, though they are very friendly.
Psyker Dredd knows you were gonna say that.

Psyker Dredd knows you were gonna say that.
Shh, my heresy sense is tingling...

Psyker

Contents

Types of Psyker