The Legion of the Damned quite literally the keepers of the flame for the Imperium. Take a look at this red hot chapter of mystery.
The Legion are afflicted by a warp contagion that destroys their sanity; as it progresses their bodies decay, but they grow in supernatural strength. Each Marine is rapidly approaching the moment when he will succumb to the contagion. Combat can initiate the final stages of the disease, in which the Marine reaches a peak in power and enters a berserker state.
Via Lexicanum:
A few scholars of Adeptus Astartes have proposed that the Legion of the Damned is a psychic backlash made real. Events such as the Drop Site Massacre 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.
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.
Prominent amongst these are the Fire Hawks, a Chapter whose history was blighted by disaster.
