The Legion of the Damned might be the closest that Warhammer 40,000 gets to the cover art of a heavy metal album. Take the brutally metal concept of a Space Marine Chapter, holy warriors in ceramite power armor wielding military chainsaws and rapid firing grenade launchers, then make them ghosts and set them on fire - that's the Legion of the Damned. But who are these mysterious warriors, and what's the story behind that heavy metal facade?
The Legion of the Damned debuted in 1988's White Dwarf episode 99, in an article by Rick Priestley, but it was 1995's Codex Ultramarines - the closest thing to a generic Codex Space Marines for Warhammer 40k 2nd edition - that introduced them in the form we recognise them today. This description is still very apt: "In times of great adversity, the Legion will come to the aid of Space Marines in battle, turning defeat into victory" - afterward, "The Legion vanishes, leaving no trace of their presence or clues as to their origin".
A short story in the voice of Ultramarines Chief Librarian Tigurius describes their arrival and appearance. Tigurius' forces are on the verge of annihilation by Orks, and - with the company chaplain dead - he leads the troops in prayer on the eve of the final battle. When he raises his head from his prayers, a host of mysterious marines have entered the fray.
"Their armor was colored black and upon it was drawn chilling images of bones and fire, and on their helms they bore skulls" - pretty gnarly couture for loyalist marines in 1995, since the Blood Angels Death Company and Mortifactors didn't yet exist in their modern forms. "As they advanced an eerie glow shrouded them and fire seemed to dance about their feet", Tigurius continues, "Like the bones of men in the torment of purgatory they were, so that they looked more like skeletons than living men. Yet not a sound did they make".
Loyalist ghost marines, then. The Legion of the Damned may have been inspired by the 1914 short story 'The Bowmen' by Arthur Machen, a tale set during the Great War in which a host of ghostly archers from the battle of Agincourt are summoned by a British soldier praying to Saint George, saving his company. Machen's story was likely inspired by the recent British victory at the Battle of Mons - and his tale would, in turn, inspire the popular myth of the 'Angel of Mons' protecting the British during that battle.
Like many things published during the first edition of Warhammer 40k, Priestley's article lays out in very explicit detail things that later lore would change to supposition and myth. Priestley identifies the Legion of the Damned as the Fire Hawks chapter. In 963.M41 the entire Fire Hawks chapter fleet, including its fortress monastery, was lost in the warp when it attempted a translation to the Crows World sub-sector. After 20 years the wider Imperium declared them dead.
Of course the Fire Hawks weren't totally gone. In this version of the lore, the Legion was driven to near madness by a warp-borne wasting disease that doomed them to an agonising death, while simultaneously heightening their strength. They fought on in service of the Imperium, but had begun to completely break down as a Space Marine chapter.
Newer versions of the lore are a lot more ghostly and less corporeal, perhaps because the agonising warp-borne disease angle became integral to the backstory for the Death Guard. The sixth edition supplement Codex: Legion of the Damned does include the suggestion that they are the lost Fire Hawks chapter, but it's now presented as one theory among several.
One theory, purported by the radical Inquisitor Quixos, is that humanity's faith in the Emperor had given rise to literal warp entities under his control, functionally speaking daemons. Some scholars believe that they are manifestations of the powers of Space Marine Librarians, drawing upon the psychic backlash of traumatic historical events like "the Isstvan massacres or the death of a Primarch" - which might explain why the Legion only appears where Space Marines are present. Radicals believe they may actually be psychic manifestations of the great mass of humanity, which - given the current Psychic Awakening in the Imperium - may explain why they've been sighted more in recent years. And a quote from an Eldar Autarch says they are simply Daemons.
The warp sickness explanation reappears as Quadrimesta's Thesis of Perpetual Martyrdom, which posits the marines have "somehow blended with the stuff of the Warp, contracting a kind of metaphysical contagion", which they are attempting to protect the rest of the Imperium from. Unlike the original version of the disease, this is hypothesised to allow them to be "reborn after a certain period of time, much as a banished Daemon can return to the service of its masters after its purgatory is complete". This connection to the Warp allows the Legion to read the Emperor's Tarot with uncanny accuracy, which is why its interventions always arrive at the last, most critical moment. But the Legion is trapped in a cycle of impurity, constantly seeking ways to strike against the foes of the Imperium to earn a redemption which its contaminated state will never permit.
While the Imperium isn't sure what the Legion of the Damned is, it has too many other things to worry about to actively pursue them as enemies. And the Legion appears to be loyal. In the battle for Cadia during the 13th Black Crusade, the Legion assisted in the assault that destroyed the power core of a Blackstone Fortress; since the fall of Cadia, and particularly during the Noctis Aeterna when the light of the Astronomicon was temporarily extinguished, they have been sighted on worlds across the Imperium.
Is Ferrus Manus in the Legion of the Damned?
Ferrus Manus, the Primarch of the Iron Hands Space Marine Legion, was slain by his brother Fulgrim during the dropsite massacre on Istvaan IV. His ghost appeared in the penultimate Horus Heresy Book, The End and the Death: Volume II, speaking to the Primarch Sanguinius - but this occurred when warp energy had entirely saturated the material realm. Fans have made some excellent fan art about a blazing ghostly version of Ferrus Manus as part of the Legion of the Damned, but there's no lore connection at all. If you want to make a conversion, though, more power to you!
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