The Reaver Titan Ferrum Mori
Even after joining with the Imperium, the Legio Mortis was looked upon with suspicion by the fledging Imperial military staff, many accusing them of being solely loyal to the Fabricator-General as opposed to the Emperor. To both demonstrate and deepen their loyalty to the Imperium, the Legio Mortis was assigned to Horus Lupercal and the 63rd Expeditionary Fleet during the Great Crusade. With the charismatic Primarch they fought over a hundred campaigns, scoring victory after victory. However rather then becoming loyal to the Emperor or his Imperium, the Legio Mortis instead developed a fierce allegiance to Horus himself. Thus it was that they willingly followed their Warmaster into treachery.
Legio Mortis War Maniple
Legio Mortis is the most notorious of the traitor Titan Legions. The Legio directly supported Horus before his daemon-possessed treachery against the Emperor. The Legio followed him willingly. When the Horus Heresy began with the virus-bombing of Isstvan III, it was the Legio Mortis which made the landings to scour the corpse-packed hives for the last survivors.
On Mars, the Legio Mortis forces supported the traitor Fabricator General Kelbor-Hal, and were the first titan legion to walk in support of Horus. These contingents of Legio Mortis, however, were all but destroyed by Legio Tempestus, save for the Hellstorm Cannon of the Imperator Aquila Ignis.
Some say that it was mutated strains of the Life-Eater Virus that was unleashed upon Isstvan III that brought about the final corruption of the Legio. When the Legio Mortis landed on Earth 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 flesh and metal lashed and their titan heads had been transformed into the visages of daemons.
The Legion breached the walls of the Imperial Palace during the siege. The Death's Heads also fought on Paramar and Tallarn.
The Legio Mortis later took part in the 13th Black Crusade of Abaddon the Despoiler.
Interesting reference on the Cy-Carnivora... Mutated flesh eating robots? Wish I could find more info on the subject.