A Psyk-Out Grenade is a very rare type of grenade used by Grey Knights Astartes, high-ranking Inquisitors and Assassins of the Culexus Temple as a potent anti-psyker weapon. Outwardly, a Psyk-Out Grenade looks much like one of the Frag Grenades in use by the many military forces of the Imperium. Its rarity and potency stems from the special additive that is laced within its explosive charge. The Adeptus Custodes reverently gather the dust that forms from the body of the Emperor of Man where He is entombed in the Golden Throne. This byproduct of the Golden Throne's arcane workings possesses a strong anti-psychic charge and can produce an effect in psykers and Warp entities similar to those produced by a psychic Blank, a being afflicted with the Pariah Gene.
The advantages of this unknown substance have not escaped the Ordos of the Imperial Inquisition, and the gathered dust is almost exclusively used for the construction of Psyk-Out grenades, although small amounts of the substance are sometimes bequeathed as holy relics to certain well-connected Cardinals of the Ecclesiarchy. When a Psyk-Out grenade detonates, the sacred dust is spread over the whole of the blast radius. Any psyker caught within it will find his or her link to the Warp violently severed, and any psychic power active in the radius will be terminated due to its sudden loss of Warp power. This traumatic experience will prevent psykers from using any of their powers for a certain time, depending on the psyker's strength, and interrupt the action of any psychic power with potentially fatal consequences for the user. The severing of the connection to the Warp is also a potent tool to be used in the fight against daemons: Lesser Daemons will be banished from realspace outright, unable to maintain their physical form, and even mighty Greater Daemons will find themselves severely weakened and liable to easy banishment.
Most Psyk-Out Grenades are used offensively, thrown at the intended target by a Grey Knight, Inquisitor or Assassin to soften it up before moving in for the kill; but they can also be used defensively, to protect the user from hostile powers. This was demonstrated by Epistolary Melmoch of the Excoriators Chapter: He sprinkled himself with the sacred dust contained in one of Certus Minor's holy relics, which knocked him unconscious, but protected him from the psychic feedback of the Khornate Cholercaust Blood Crusade that would otherwise have slain him outright.
- Assassinorum Executioner Force: Rulebook (7th Edition), pg. 31
- Codex: Assassins (2nd Edition), pp. 7-8
- Legion of the Damned (Novel) by Rob Sanders, pg. 77