Legio Mortis - Horus Heresy: Ironically, the Death's Heads would become corrupted by the Chaos God of decay, Nurgle -- a deity whose undead servants they had once fought against on Davin's moon, where Horus had been mortally wounded.After the Warmaster's "miraculous" recovery from his wounds following his corruption by the Chaos Gods in the Temple of the Serpent Lodge, he began his rebellion against the Imperium by unleashing Life-eater Virus Bombs upon the world of Isstvan III, seeking to destroy elements of the Traitor Legions that were still steadfastly devoted to the Emperor of Mankind.Elements of the Titan Legion had already descended to the surface where what few elements of the Legio that were not trusted were quickly purged.It was to the Legio Mortis in their sealed war engines that Horus' plan had given the coup-de-grâce to the few Loyalist Astartes expected to survive the initial virus bombardment, but as matters transpired, what was to be an extermination to scour the survivors evolved into a wholesale battle.Horus ordered the full Titan Legion deployed to smash the husks of the Isstvanian cities and break the Loyalists' resistance, and for the first time the Legiones Astartes met the Adeptus Titanicus in open conflict.Some claim that mutated strains of the Life-Eater virus brought about their final corruption, while others say that the madness wrought by the sight of billions of putrefying corpses turned the Legio's faces to Chaos.When the Horus Heresy became a full-blown insurrection, on Mars the traitorous Fabricator-General Kelbor-Hal of the Mechanicum decided to take the Warmaster's side in the conflict. The Legio Mortis supported the fabricator-general, being one of the first Titan Legions to fight in the service of the Ruinous Powers.Open hostilities erupted all over the Red Planet as the civil war within the Mechanicum that became known as the Schism of Mars began. Loyalist and Traitor forces alike brought forth forbidden technologies to use against one another.The Dark Mechanicum-aligned Legio Mortis eventually fought the Legio Tempestus in open warfare near Mars' Magma City, but these contingents of Death's Heads Titans were wiped out by the Loyalist Titan Legion that defended the forge city.For the remainder of the Horus Heresy, the Legio Mortis proved a constant companion to the Warmaster and his forces. From Isstvan V and the world of Molech to the Beta-Garmon System and the walls of the Emperor's palace on Terra itself, the god-engines of the Legio Mortis brought ruin and death to all assembled beneath the Imperium's banner.Records from innumerable battles during the Horus Heresy report the presence of Death's Heads Titans, yet their prolific acts of treachery did little to diminish the strength of the Legio Mortis, for the Warmaster ensured they remained well-supplied.By the time the Legio Mortis landed on Terra to take part in the siege of the Imperial Palace during the Siege of Terra, its Titans had been warped and mutated beyond all recognition -- their adamantium hulls pockmarked and bubbling with fetid vapours.The twisted flesh and metal of great tentacles lashed to and fro, spiked tails whipping about. The once-noble skull-faced heads adorning the Legio's Titans had now been transformed by the eldritch powers of Chaos into rancorous visages of drooling Daemons filled with malice, their engines roaring like angry beasts.The Warmaster granted the Legio Mortis the singular honour of breaching the outer walls of the Imperial Palace. Using Wreckers, Power Rams, Warp Missiles and their own fiendish bravery they managed to accomplish this near-suicidal task despite suffering the loss of over thirty Titans in one evening of intense fighting.The walls of the Imperial Palace finally broke under the onslaught by the Death's Heads Titan Legion -- the huge Warlord-class Titans of the Legion smashed their way through the last few metres of the bastion and unleashed a flood of Traitors into the inner courtyard of the Palace.But despite their efforts, ultimately the Siege of Terra failed and Horus was laid low by the Emperor Himself. The remnants of the Legio Mortis fled from Terra hunted and pursued, system by system, to the Eye of Terror.There, where the Warp and realspace overlap, time has flowed differently for the Death's Heads. They remain trapped in servitude to the Ruinous Powers, fighting the Long War against the forces of the Corpse Emperor that began ten millennia before, building their strength and testing the Imperium's defences for the time when they shall return and wreak terrible vengeance on all living things for the defeat of their beloved Warmaster.