A doomscryer is a type of powerful, precognitive psyker who is stationed alongside the astropaths of the Imperial Palace to aid the Adeptus Custodes, the Emperor of Mankind's praetorians, by searching the tides of the Warp for warnings or indications of future threats to the safety of the Emperor. When these threats are discerned, the Custodes despatch their forces to avert the coming disaster or eliminate the threat by any means necessary.
The doomscryers also use their precognitive and clairvoyant abilities to determine the identity of those individuals -- through word, thought or deed -- who possess the ability to avert these disasters before they grow grave enough to threaten the occupant of the Golden Throne. Those individuals who are fated to be saviours are accorded the high honour of a protective detail of Custodians drawn from the Aquilan Shield, so that the Custodes can ensure that a significant Imperial asset survives to carry out the fated actions in the Emperor's defence.
Corruption of the Doomscryers
However, by the late 41st Millennium, the Custodians had become overreliant on the abilities of the doomscryers. A time of strange omens and ominous whispers engulfed Terra that was remembered by the Custodes as the Time of Madness, beginning with the disappearance of the notoriously conservative Captain-General Galahoth. Battling the stagnation of Galahoth's rule, the Adeptus Custodes found themselves facing a shocking increase in cult activity -- both heretical and xenophile -- throughout the Sol System. Doomsday sects triggered queue-wars between the pilgrim tribes within the Imperial Palace itself, and the Custodians were forced to exercise their authority in the bloodiest fights they have seen in centuries.
Reports from the Dark Cells cited a growing sense of agitation amongst the hidden inmates, and numerous support Servitors had to be destroyed by the Shadowkeepers after they exhibit sudden, violent madness. Worse followed when daemonic possession was revealed amongst a sub-sect of the doomscryers themselves, though not until the false predictions of the fallen psykers had aleady sent Captain-General Andros Launceddre to his death at the Battle of the Gilded Pyre. It was amidst this climate of spiralling paranoia and danger that Captain-General Trajann Valoris was elevated to command the Ten Thousand, and he wasted no time in taking the steps necessary to regain ironclad control of Terra's defences.
- Codex: Adeptus Custodes (8th Edition), pp. 24, 31