Grey Knights

The Grey Knights are a secret and mysterious Chapter of Space Marines specifically tasked with combating the dangerous Daemonic entities of the Warp and all those mortals who wield the corrupt power of the Chaos Gods. They were created by the Emperor with the aid of Malcador the Sigillite at the time of the Horus Heresy to serve as Humanity's greatest weapon against the threat posed by the existence of Chaos. They have the honour of being implanted with gene-seed engineered directly from the genome of the Emperor Himself.

The Grey Knights act as the military arm or "Chamber Militant" of the Ordo Malleus, the Daemonhunters who form the oldest branch of the virtually omnipotent Inquisition. The Grey Knights' fortress-monastery, the Citadel of Titan, is based on Titan, the largest of the moons of the gas giant Saturn in the Sol System. It is kept as a private preserve of the Inquisition.

The existence of the highly secretive Chapter is virtually unknown outside of the Inquisition and the highest echelons of the Imperial adepta, and is a well-guarded secret enforced by mind-wipes and even assassination of Imperial citizens if necessary.