Dark Mechanicum - Horus Heresy: Aside from the Human component in the Traitors' ranks, the powers of the Mechanicum and their kin were also of great importance to the war. Within the ranks of the burgeoning "Dark Mechanicum," as they became known, could be found powerful elements of the Martian Priesthood, the Ordo Reductor and the Legio Cybernetica, along with many of the feared Myrmidon Destructor Cults and a number of sub-cults which had operated for long years on the edge of tech-heresy, all drawn together by Kelbor-Hal of Mars.With them had come the support of more than half of the legios of the Adeptus  Titanicus as well as dozens of allied Knight houses, and for much of the Heresy Mars itself was lost to the Loyalists, while the output and military power of Forge Worlds such as Sarum, Voss, Cyclothrathe and Stygies VIII had declared for the Traitors, with others such as Anvilus, Incaladion and Ryza were paralysed by civil war.Much as with Sarum and Cyclothrathe, other "Renegade" Forge Worlds distrustful of Mars that the Warmaster Horus had enticed into his cause with the lure of petty empire-building and freedom of experimentation, Horus offered new treaties of alliance whose accords the varying ruling Forge Worlds' Mechanicum Synods would find more to their liking than the old.By this pact these Forge Worlds would serve the Warmaster's "new Imperium" just as it had the old, but where they had been used and bled dry callously in the past by a master who cared nothing for their power or prosperity, under Horus they would flourish and be rewarded.Gone would be the petty restrictions of the Emperor's technological edicts -- blocks put in place to avert the spectre of the terrors of Old Night -- and gone would be the dead hand of Machine Cult dogma, if they so desired, so long as they offered up their weapons and war machines to the Traitors' cause with abundance.The details would prove more torturous and elaborate to finalise, but in secret an agreement was reached, even while in false faith the lords of these Renegade Forge Worlds continued to secretly deal with the Regent of Terra Malcador the Sigillite's emissaries to the contrary. Many archmagi sought to play one side against the other, right to the bitter end.The Dark Mechanicum wholeheartedly supported Horus during the start of the Horus Heresy and they participated in the attacks against the Loyalist Space Marine Legions on Isstvan V where they used dark and forbidden knowledge to help destroy the Loyalist Astartes. Later, the Dark Mechanicum, led by Kelbor-Hal, unleashed the terrible civil war upon the Mechanicum known as the Schism of Mars in a bid to restore the autonomy of the Mechanicum from the Imperium of Man.This effort ultimately failed when Horus was slain at the end of the Horus Heresy during the epic Siege of Terra and the Loyalist elements of the Mechanicum succeeded in driving their Chaos-corrupted brethren from the Red Planet. Many of these so-called Hereteks of the Dark Mechanicum fled into the Eye of Terror alongside the other Traitors after the campaigns of the Great Scouring, when the Imperium recovered most of the territory across the galaxy that had been lost to Horus' Traitor forces.