The Culexus Temple is one of the darkest and most feared chapters of the Officio Assassinorum, and its assassins have been described by the Aeldari as nothing short of pure evil. His agents are skull-helmed deathbringers, nearly invisible terrors whose mere touch can snuff out the soul of the victim. Even among the upper echelons of the Officio Assassinorum, this temple is always viewed with extreme caution. This is not due to the way the Culexus operate, nor to any particularly heinous methods they use to kill their targets. That's because of the assassins themselves, because they're carriers of the pariah gene.
First and foremost, a would-be Culexus assassin is chosen because he possesses a very rare genetic defect, something at once unique and yet utterly despicable to the Empire. Every living thing has some kind of presence in the warp where the soul is reflected from the real universe.
For non-psionics, this is usually little more than a tiny spark. For psykers, however, their soul blazes like a glowing beacon that can be seen by other psykers and creatures living in the warp. The more powerful the psyker, the brighter his soul burns in the realm of the Empyrean. Culexus Assassins are true terrors because they have or appear to have no presence in the warp - there is simply a void in their place. They are soulless in every way.
This extremely rare genetic defect affects fewer than one in a billion people. Those suffering from this condition are known by the name Pariah or Untouchable and encountering one is something no normal person can ever forget. Her cold, calculating gaze draws attention to her eyes; Pools of blackness that seem to draw you into a pit of nothingness. The mere presence of a pariah leaves those around them fearful and increasingly paranoid as an all-pervasive sense of the wrong causes skin to tingle and senses to rebel. Some may complain of an unbearable headache or ever-increasing senseless panic, their heart pounding and their breathing erratic.
For a psyker, this feeling is infinitely worse, manifesting in terrified panic attacks, seizures, and the monstrous feeling of being suffocated or drowned.
Such terrible phenomena make pariahs outcasts in human society. Yet it is precisely these powers that the Culexus Temple refines and amplifies to create its fearsome assassins. As a result, they can emit negative psychic energy that is deadly to psykers and warp beings alike.
Culexus Assassins are typically used against psychic targets such as heretics, Asuryani warlocks, or even demons.
However, Culexus Assassins were turned into their foul pariah servants by the Necrons by transferring their consciousness into Necrodermis bodies.