Blank

Blanks can be smexy too...

Blanks (also known as Untouchables or Pariahs) are a extremely rare breed of human psyker, even in the explosive populations of the Imperium, and average about 1 or 2 on the average planet of 10 billion inhabitants, with many planets never (or go centuries without) having one be born at all. They function as anti-psykers, having a warp-becalming presence which fucks with people's souls. This makes them seem unnatural to ordinary people in an indescribable way, and are furthermore an anathema to psykers because they shut off their powers and warp sense. This unfortunately has an effect on those people more susceptible to the warp, causing pangs of sickness and confusion. They naturally and perpetually cause a preternatural sense of revulsion in all living things, since all living things in Warhammer 40K have at least a little bit of a warp presence. Most tend to die at a young age due to social ostracism or lynching, due in no small part to the fear, horror, and a sense of paranoia in everything around them.

Blanks are supposedly seeded into humanity by the Deceiver, a C'tan god, as a way to counter the powers of the warp in the future. This was done by abducting humans, and genetically modifying them by inserting a "Pariah Gene". However, this was the pre-5th edition Necron fluff; with the release of the 6th edition's TOMB KINGS IN SPEHSS fluff, this is now pretty dubious since humanity hadn't even evolved yet during the age of the Necrontyr. Though, like with all oldcron fluff, it could be easily explained by the results of an escaped Deceiver shard, or even mega-shard (a few shards merged together). The more recent fluff gives an alternate spin on this, with some characters suggesting that humanity was unique in that Blanks occurred naturally as a part of their race. Whether those characters (in universe) knew about the oldcron pariah gene is up in the air.

The ultimate fate of most Blanks is to either be picked up by the Black Ships and turned into Culexus Assassins, or get killed by the overzealous citizens of the Imperium due to their (admittedly ten-amplified) rampant phobia of anything vaguely inhuman, or end up fed to the Kroot by the Tau so they can have anti-psykers of their own. They also used to be turned into Necron Pariahs if ever the Necrons managed to capture a Blank. However, with the fluff re-write, this apparently is no longer the case. Asdrubael Vect kidnapped a few Nulls from an Inquisitor stronghold once. Apparently, Something deep underneath the city of Commorragh was trying to get out, and he believed that creatures disruptive to the Warp could help if connected to the proper devices. Horrible, horrible devices, if the Dark Eldar have anything to say about it. Also, "underneath" relative to what?

Nulls

Nulls are another, less soul-rending version of Blanks. Instead of actively repelling the Immaterium, Blanks simply have no presence in the Warp at all. In terms of the Warhammer fluff, the warp presence of all mortals is literally their soul. This means that Nulls have no soul (and Blanks have an anti-soul?). While nowhere near as powerful as Blanks, Nulls are several times more common than Blanks. They are useful tools for the Inquisition and the Ecclesiarchy, on account of their more subtle "powers", but they are common enough that neither faction actively scoops them up with Black Ships, unlike psykers or Blanks. Blanks are harder for most living creatures to notice, apparently because everyone is a little bit psychic or something. Like Blanks, psychic powers aren't as effective on Nulls, but unlike Blanks, Nulls can't just shut down a psyker. Instead of hating or fearing them, most people find Nulls just a bit off, which means that most Nulls aren't actively lynched before the Inquisition can make use of them. Nulls also fuck with daemons in a number of ways, which makes them great for daemon hunting.

(the above isn't technically wrong but the fluff has been wildly inconsistent in the differences, if any, between Blanks, Pariahs, Blacksouls, Nulls et al. The closest thing to clarification is during Abnett's Pariah novel where a couple of Word Bearers complained that the protagonist, a Pariah, was barely even a Blank, let alone a Blacksoul.)

Sisters of Silence

The Sisters of Silence were one of the most prominent users of blanks in the entirety of the Imperium. They were formed by Malcador the Sigillite during the early days of the Great Crusade to aid the forces of the Imperium against some now-unknown xenos races (who weren't the Eldar, apparently) who had psykers of their own and used them against the fledgling Imperium.

During the Horus Heresy the Sisters of Silence proved instrumental in defeating the Traitor Legions and their large groups of psykers. They were present during the Burning of Prospero and were vital in silencing the legions' ranks of psykers. They did however suffer heavy losses, but in the turmoil that followed the Heresy lost a lot of momentum. It is unknown now whether the organisation still exists or not. A shame, because they could be very valuable against the enemies of the Imperium now. Nope, they're back with friends.

Hope for the Necrons, or, The Possibility of Pariahs Returning ...MAYBE

This article or section involves Matthew Ward, Spiritual Liege, who is universally-reviled on /tg/. Because this article or section covers Ward's copious amounts of derp and rage, fans of the 40K series are advised that if they proceed onward, they will see fluff and crunch violation of a level rarely seen.

In the current Necron fluff starting from the 7th Edition Codex, Illuminor Szeras managed to capture a Culexus Temple Assassin during a surprise incursion against a world held by Chaos (see "993.M41: A Dark Experiment" in the current Codex) and trolls the High Lords of Terra into sending more of them (the old farts respond by sending an entire Assassin Execution Force, though it's not specified if those were composed of Culexus Assassins or other types of Assassins).

This may be something of a good thing, since a lot of people actually liked Pariahs and were upset after Matt Ward removed them in favor of the Lychguard.

Reminder: Necron Pariahs were once Blanks that were converted by the 'Crons as anti-psyker cyborg shocktroopers armed with Warscythes and somewhat as a retinue for Necron Lords. Pariahs were introduced in the 3rd Edition Codex Necrons as an Elite Choice, but were removed in the 5th Edition Codex.

The above incident with Szeras and the Culexus may even be made into a new Pariah Origin Story. Even back in the times when GeeDubs were still cool (and their site was still more than a catalogue, and had hobby ideas and more), there was a scenario telling the origin story of how a Necron Pariah came into being: the Necrons had to escort to safety a prototype Pariah (who was de-facto a converted Culexus Assassin who had capabilities of both Culexus AND Pariah and kept his assassin wargear in addition of getting a Warscythe) while protecting him from an Inquisitor and his army.