A Null-amp Collar was a type of anti-psychic device dating back to the Dark Age of Technology utilised by Imperial and Traitor forces during the Great Crusade and Horus Heresy eras. Its user possessed the ability to nullify Warp powers in a given area of effect around the collar.
Recovered as part of a joint operation by the Iron Hands and World Eaters Legions, these devices are the product of a human civilisation far removed from that which survived on Old Earth, now long forgotten. In form they are thick collars of black silicate material, dense and seemingly without obvious components.
When fitted to a living being, they project an aura that pacifies the churning of the Immaterium in the vicinity of the device, though at cost to the focus and mental acuity of the bearer.
Before its disbandment, the fledgling Librarius Project of the Space Marine Legions held several examples of these devices for study as potential anti-psyker weapons, and these most likely found their way into the hands of the Legions after that body's disbandment at the Emperor's orders following the Council of Nikaea.
In later years, the few surviving examples of this technology have been used to engineer a "Psychic Hood" to augment the ability of a human psyker to combat Warp powers without the debilitating cost of the original devices.
- The Horus Heresy - Book Eight: Malevolence (Forge World Series) by Neil Wylie and Anuj Malhotra, pp. 300-301