Ordo Malleus - Imperial Secrecy: To protect its citizens from the insidious temptations of Chaos, the Imperium of Man long did its best to hide the existence of the Chaos Gods, Daemons and the Chaos Space Marines from public knowledge. Only certain Space Marines, Sanctioned Psykers and the members of the Inquisition were permitted to know the Imperium's darkest secret.It was long Inquisitorial policy to mind-wipe even members of the Adeptus Astartes, including entire Chapters in some cases, after exposure to the Daemonic. All others are either put to death after exposure to the reality of Chaos to protect the Imperium from their possible corruption, or if they have been a valuable servant to the Imperium, they are allowed to live but required to undergo memory modification or even, in extreme cases, a mind-wipe.This is a policy that has been in place since before the Emperor was interred within the Golden Throne, when only He and His primarchs knew that the Warp contained intelligent entities capable of possessing individuals in realspace.But even the Emperor did not reveal to His primarchs during the Great Crusade the full truth that the Warp was not just a seething cauldron of psychic energies inhabited by entities similar to xenos, but was actually populated by malign intelligences akin to the supernatural beings of ancient Human myth and superstition. He chose not to explain that the Empyrean was dominated by the Ruinous Powers and their Daemonic servants, for fear that this knowledge alone would lead too many of the primarchs to take actions that would inevitably result in their corruption.To fight a Daemon army is to fight a twisting tornado of unreason and despair that forever changes those who must confront its horror. As such, the Imperium believes that it cannot allow the knowledge that such foes actually exist to spread, since even the simple knowledge of Chaos' existence may mark the start of an individual's fall to damnation. The Human survivors of conflicts with the Daemonic were invariably confronted by the agents of the Inquisition and mind-wiped, quarantined for life in forced labour camps or even -- in extreme cases -- made the subjects of a worldwide Exterminatus event.Over the aeons, the galaxy has witnessed Warp-based catastrophes and Daemonic incursions beyond counting. Since the inception of the Inquisition after the Horus Heresy, even the fact that such a thing is possible is deemed too dangerous for the citizens of the Imperium to know, for such knowledge breeds heresy as surely as a flyblown corpse breeds maggots.Because of this, the vast majority of knowledge concerning Daemonic incursions has been eradicated from extant Imperial public records by the actions of the Inquisition. What is known is recorded only in proscribed Imperial texts and heretical xenos scripts that the Inquisition has yet to destroy.However, in the wake of the opening of the Great Rift at the start of the Era Indomitus, this policy of secrecy has been somewhat relaxed, at least for the Adeptus Astartes, due to necessity. Before the opening of the Great Rift, the vast majority of Astartes were expected to be as ignorant about the existence of Daemons as any other citizen of the Imperium. In truth, it was hard to find an Astartes who had not fought Daemons at some point by the end of the 41st Millennium. Yet the Inquisition in the Time of Ending was well-known to mind-wipe entire Chapters after certain incidents, though not every Chapter was willing to submit. Some like the Space Wolves resisted any intrusion on their traditional autonomy forcefully and retained their institutional knowledge of how to fight Daemons.But in the Era Indomitus, with the galaxy now riven in half by the birth of the Cicatrix Maledictum, Daemonic incursions are so common, and Space Marine responses so necessary, that suppressing the knowledge of the existence of Daemons among the Astartes has simply become pointless.