Dark Mechanicum

The first leader was Fabricator General Kelbor-Hal, and the deal was sealed by Horus’s emissary Regulus, who unlocked the Vaults of Moravec — a repository of the forbidden (AI, Men of Iron, warp weapons) — in exchange for a promise to lift the Imperial ban on Abominable Intelligence. Into the ranks of the Dark Mechanicum stepped Ordo Reductor, Legio Cybernetica with its legions of battle automata, Ordinatus engines and corrupted Skitarii, while the forges of Xana, Cyclothrathe, Sarum, Urdesh, Estaban and Retlaxi fed the traitors’ war. From Isstvan V and the Schism of Mars they went on through the Titan-slaughter of Beta-Garmon, Molech, Tallarn and the Siege of Terra. Mars was blockaded by the Imperial Fists; after the death of Horus the loyalist Mechanicum expelled its corrupted brethren, and the magi-Hereteks fled to the Eye of Terror in the Great Scouring.

It won Mars but was blockaded; after the death of Horus the loyalist Mechanicum expelled its corrupted brethren, and many Hereteks fled to the Eye of Terror.