Imperial Armour 13: War Machines of the Lost and the Damned (2014), p139 — The Plague Titans of Legio Mortis

The very name of Legio Mortis has long since become a foul taint to the defenders of humanity, uttered only with fear and loathing. Their infamy stretches back 10,000 years to the time of the Horus Heresy, Legio Mortis having fought for the Warmaster in a hundred campaigns during the Great Crusade, their loyalty long since pledged to him rather than the distant Emperor. When Horus began the Heresy by virus bombing the rebellious world of Isstvan III, the Death's Heads made the landings to scour the corpse-packed hives for the pitifully few survivors. Some say that mutated strains of the virus brought about their final corruption whilst others claim the madness inspired by the billions of putrefying corpses turned their souls to Chaos.

Certainly when the Legio Mortis landed on Terra to besiege the Emperor's Palace, they were warped and mutated almost beyond recognition. The adamantium skins of their Titans were pocked and bubbled with foul effluvia, great tentacles of twisted flesh and metal lashed, and spiked tails whipped back and forth. Their Titans' heads had been transformed into drooling daemonic visages filled with malice, and their engines roared like angry beasts. Horus granted the Death's Heads the honour of breaking through the outer walls of the palace and with wreckers, power rams, warp-tainted missiles and their own fiendish obsession they did so, despite suffering the loss of over thirty Titans in one night. But despite their efforts, the siege failed and Horus was defeated.

The remnants of the Legio Mortis fled and were hunted and pursued, system by system, to the Eye of Terror. There, where the Warp and realspace overlap, time has flowed strangely for the Death's Heads. They remain trapped in servitude to the gods of Chaos, fighting a war that ended ten thousand years ago, building their strength and testing the Imperium's defences for the time when they shall return and wreak terrible vengeance upon the galaxy.