'Chapter fights in complete, eerie silence'

That is all you need to know.

"Of all the legends of the Imperium, one of the strangest is that of the Legion of the Damned. For those who know where to look, there are many corroborated accounts of these otherworldly warriors, their unexpected appearances upon a desperate battlefield, and their sudden and inexplicable disappearance at the battle's end."
— Inquisitor D. Merloriac, Ordo Chronos

Founding
Cursed 21st Founding (36th Millennium)
Successors of
Ultramarines (allegedly); the Legion is a remnant of the Fire Hawks
Successor Chapters
None Known
Number
Unknown
Primarch
Roboute Guilliman (allegedly)
Chapter Master
Unknown
Homeworld
Zhoros (Destroyed)
Allegiance
Imperium of Man
Colours
Black with bone and flame decorations

The Legion of the Damned are a mysterious band of Space Marines who appear unbidden when all hope seems lost, striding forth from their hidden netherworld to bring retribution to the Imperium's foes. Once their bloody work is complete they vanish into myth once more. They are a legend, whispered in secret by a suspicious few, that the Emperor has the loyalty of a force from beyond reality. The Legion is an army of vengeance and of flame, a brotherhood of warriors who appear at first glance to be Space Marines but quickly prove to be more lethal than even those most formidable of transhuman warriors. These warriors are variously said to dwell in the spaces between this world and the realm of the Warp, in the hopes and dreams of desperate men, and in a pocket of reality eternally lit by the fires of hatred. Rare is the man that has witnessed the sudden violence of their attack, rarer still those with the strength of will to keep their sanity intact afterwards.

In truth, the Astartes that make up the self-proclaimed Legion of the Damned are remnants of the lost Loyalist Fire Hawks Chapter. They fight as a single unit and often appear mysteriously to aid an Imperial military force at a moment when they are about to be overwhelmed by the foe. The Legion's Space Marines are afflicted by a strange Warp contagion that destroys their sanity; as it progresses, their bodies decay physically at the cellular level, but they grow in supernatural strength and the ability to use the power of the Empyrean against the Emperor's foes. Each Astartes of the Legion is rapidly approaching the moment when he will finally succumb to the contagion. Combat can initiate the final stages of the disease, in which the afflicted Space Marine reaches a peak in physical power but also enters a permanently berserk state that ends with his death.

The haunting legend of the Legion of the Damned is known across the galaxy, and even nonbelievers speak of such things in hushed tones. These silent warriors are Space Marines in appearance, their black armour adorned with images of bones and fire, yet they are not of any Chapter recognised in the Imperium. Most eyewitnesses dispute even the Legionaries' mortality, for an eerie glow suffuses their sable armour and a halo of ghostly fire dances about their feet. There are many corroborated accounts of Legionaries of the Damned enduring firepower that would annihilate mortal men. Among them is a report from the Keyan battle zone, where a handful of Astartes of the Legion of the Damned were witnessed walking unharmed from the Volcano Cannon blast that claimed four Predators and several squads of Space Marines. The Bolters carried by the Legionaries, though in aspect no different to those borne by other Space Marines, discharge flaming projectiles that can pierce the strongest armour. Nothing, not Chaos Chosen, rockcrete bastion, nor boiling lava can stay the spectral wrath of the Legion of the Damned.

The nature and origin of the Legion of the Damned is shrouded in mystery and myth. Some rumours suggest that they are the survivors of the lost Fire Hawks Chapter, transmuted by the Warp Storm that claimed their vessel. Others imply that the Legion are an extension of the Emperor's will, time-lost saviours, or even the vengeful spirits of Space Marines slain in the Imperium's many wars. In the course of their investigations, several [Inquisitors] have tried to capture or intercept the Legion of the Damned, but all have failed; events always inexplicably conspire to prevent the Inquisitors from getting close to their quarry, leaving them nothing.

Who, or what, guides the Legion of the Damned is an enigma to all save the beneficent Emperor Himself. They appear only in times of great need, coalescing from the fires of a desperate battlefield to turn a disaster into victory. Regardless of the conflict, the Legionaries fight with a chill precision that few mortal warriors can match, sweeping enemy positions like vengeful ghosts. Terror is their harbinger and oblivion their gift. After the battle, the Legion of the Damned depart as suddenly as they arrived, leaving only the bodies of the slain and wonder in their wake.