A few scholars of Adeptus Astartes history have intimated that the Legion of the Damned is a psychic backlash made real. Events such as the Isstvan massacres or the death of a Primarch were so traumatic they left a permanent scar on the psyches of all of the First Founding Legions, so these theories run, a scar so deep it has permeated not only the Chapters that came after them but hundreds of successor Chapters since that tumultuous time. The Space Marines that have remained loyal to the Emperor since then have a deep-seated belief that those who betrayed them should be made to pay for their transgressions with fire and fury. It is quite possible that the Librarians of the Adeptus Astartes tap into this belief and make it real, whether by accident or design, resurrecting the ghosts of their fallen brothers and giving them a chance to wreak their terrible revenge. These scholars believe it is a Chapter’s burning conviction that the guilty must be punished that gives the phantoms of the Legion a physical form.

There are radicals amongst the Librarius who have a similar theory, though they believe the Legion of the Damned is a kind of consensual hallucination, and occasionally even manifestation, conjured by Mankind as a whole. They believe that the subconscious power of hope is so strong, that when combined with the nascent psychic abilities that lurk within Humanity’s minds and a stimulus of imminent death, it can potentially be made reality. Central to this theory is the idea that though a single man may have but the tiniest shred of spiritual or mystical ability, the uncounted trillions of humans that populate the Imperium all contribute to a kind of psychic reservoir that can be tapped into in extremis, even by those who do not realise they are doing so. The Adeptus Astartes have come to embody both hope and terror to a beleaguered Imperium, and the form of deliverance and vengeance alike is the broad silhouette of the Space Marine.

A darker premise, held by Inquisitor Quixos before he was slain by Gregor Eisenhorn, is that Humanity’s omnipresent faith in the Emperor has bestowed upon the Master of Mankind a caste of supernatural agents. Just as the Dark Gods of Chaos have daemonic servants made of the same soul-stuff that sustains them, Quixos theorised that the Legion of the Damned were shards of the Emperor’s will given form. Furthermore, the Inquisitor’s writings posit the idea that these ‘Engels Mortis’ could take a number of forms, just as the Daemons of the Ruinous Powers range from the diminutive to the colossal, and that their greater forms have yet to be recorded. It is said that when the deserving and the devout find themselves on the brink of death these spectral warriors will emerge, blazing with the fires of the Emperor’s immortal will. However, Quixos maintained, there is plenty of evidence that almost every Legion appearance has happened in an area where the metaphysical barrier between realspace and the Warp is thin, suggesting a kernel of truth behind his claims that the Legion are creatures of the Immaterium. Quixos’ premise was widely decried as heresy at the time, and after his death, purged from the Ordo Malleus’ records. Yet it somehow lingers on.

Another belief whispered amongst the hidden orders of the Adeptus Astartes is that the Legion of the Damned were once a Chapter of Space Marines, a force that has been claimed by the fires of the Warp and reborn into a more sinister form. After all, across the Imperium’s long history, several contingents of Space Marines have made translation into the Empyrean and emerged changed as a result, or not emerged at all.

The simplest theory as to the Legion’s nature, and therefore perhaps the most credible, is that its warriors are simply ghosts. They are the spiritual echoes of once-great Adeptus Astartes whose hatred for the enemies of the Imperium was so powerful that it transcended mortal limits and became something eternal. If this is true, then the Legionnaires may come from a great many Chapters, their insignia and heraldry subsumed by their supernatural rebirth and replaced by icons of doom so that they are linked together by their own damnation. The few reports of those who have seen the Legion of the Damned unhelmed speak of ghostly pallor, sunken skin, and hollow eye-sockets. Every supernatural occurrence, every glitch in reality that is attributed to these mysterious warriors adds credence to this theory. Perhaps those who know of their existence prefer to imagine the spectral Space Marines as revenants, for what mortal mind could possibly stand a life bound to the hell dimension of the Warp and still remain sane?