Blank

Blanks 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. They function as anti-psykers, having no warp presence of their own. This makes them seem unnatural to ordinary people in an indescribable way, and anathema to psykers because they shut off their powers and warp sense. This unfortunately has an effect on some people more susceptible to the warp, causing pangs of sickness and confusion. 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. 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).

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 had psykers of their own and used them against them.

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, and in the chaos 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.

The ultimate fate of most Blanks is to either be picked up by the Black Ships and turned into Culexus Assassins, to get killed by the overzealous citizens of the Imperium due to their rampant phobia of anything vaguely inhuman, or to be 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, however, had kidnapped a few Blanks (called Nulls in the source for some reason) from an Inquisitor stronghold. Apparently, Something deep underneath his city is trying to get out, and he believes that creatures disruptive to the Warp can help if connected to the proper devices.

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.

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 many more, there was a scenario telling the origin story of how Necron Pariahs came into being as 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 a Inquisitor and his army.