Blank

Blanks can be smexy too...

"Even if you possess the eyes of God, you can't possibly dodge an attack coming from somewhere you can't see!"

– Buccaneer, Fullmetal Alchemist: Brotherhood

"Having an aura of menace is like having a pet weasel, because you rarely meet someone who has one, and when you do, it makes you want to hide under the coffee table."

– Lemony Snicket

"Pariahs? ... They just kind of up sticks and left, I guess."

– Kitten

Blanks (also known as Untouchables or Pariahs) are a extremely rare breed of human psyker, even in the explosive populations of the Imperium; they 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 an anathema to psykers in particular 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. Blanks 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 were 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 - though it does beg the question of what exactly constitutes "underneath" relative to Comorragh (and potential horrific implications thereof).

...And then it turned out in the Grim Darkness of 40k, even chaining a bunch of Pariahs and Nulls to your fancy gate to keep out the Warp and The Giant Purple Slut won't save you. Instead, your wall of Blanks turns into paint or, somehow, actual daemons.

Another confusing knot in the fluff is that humans (and Astartes) can be rendered soulless by technological, psychic, sorcerous, or daemonic means. It doesn't appear to be induce any typical effects of being a Pariah, but daemons despise them because a soulless being is utterly useless to them. Being soulless doesn't necessarily automatically kill them either, although this is presented inconsistently too. Whether other races can be rendered soulless the same way is unknown.

Nulls

Nulls are another, less soul-rending version of Blanks. Instead of actively repelling the Immaterium, Nulls 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.)

Actually the difference is probably in the grading. See, the Imperium uses a 24-point scale known as The Assignment to classify just how powerful a psyker is. Its based on the Greek alphabet, with Rho and Pi being baseline humans. The further up the scale you go (all the way up to Alpha, with some freaks of nature like Big E, Magnus, Malcador and others breaking it as Alpha Plus) the more powerful the psyker, however Rho is actually smack bang in the middle of the Greek alphabet, and there exists ranks lower than Rho (all the way down to Omega) which classifies how psychically inert a person is. Therefore it's likely that anyone below Rho is technically a Pariah, but specifically a Null is someone with a Sigma or Tau rank, a Blank is ranked as Upsilon, Phi, Chi, or Psi, and a Blacksoul is an Omega.

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.

According to the newer lore, the Sisters were never "officially" disbanded in the wake of the Horus Heresy. However they did lose prominence to the point where they received little to no support from the Administratum. By M32, only one major convent was found, though it was enough to assist in ending the threat of The Beast.

After that they once again disappeared into anonymity, with a small number being absorbed to man the Inquisition's Black Ships, though the vast majority dispersed to found secret convents to continue the task that the Emperor founded them for. Tannau Aleya, one of the point-of-view characters of Watchers of the Throne, came from one of the latter.

In the wake of the invasion of Fenris by the Thousand Sons and its eventual devastation by Magnus the Red, the scattered Sisters of Silence were quietly (heh) summoned to Terra on the order of the current Captain-General of the Adeptus Custodes Trajann Valoris. There they returned to the public eye of the Imperium when they help save Roboute Guilliman from Magnus, and aided in the defense of the Second Battle of Terra that came after.

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

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.

As a 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.