Culexus' presence

The helm is only partially effective at muffling the Assassin’s null-presence, even when operating at full power; most people can barely stand to be in the same room as the wearer due to the nauseous, head-spinning effect that is the trademark of a Culexus Assassin’s presence. When it is switched off altogether, the mayhem that ensues is terrifying to behold. Victims in close proximity may flee in wild-eyed terror, or freeze in place, hyperventilating and shaking uncontrollably. Others collapse, clutching their heads as bloody froth oozes from tear-ducts and gaping mouths. Psykers scream in horror as their connection to the Warp is severed, the Assassin’s powers snuffing out their own like the flame of a candle.

"Dataslate: Officio Assassinorum"