The Mechanicum was the vast priesthood of Mars, the guild of tech-savants and machine-worshippers whose mastery of the arcane sciences underpinned the entire Imperium. Bound to the Emperor by the ancient Treaty of Mars, they supplied the weapons, warships, and war-engines that made the Great Crusade possible, worshipping the Machine-God even as they served humanity's temporal master.
Their domains stretched across the forge worlds of the galaxy, each a fiefdom of belching foundries and hooded adepts who saw divinity in the workings of technology. From the Mechanicum came the towering war-titans of the Legio Titanicus, the automata of the Legio Cybernetica, and the sacred knowledge salvaged from the ruins of the Dark Age of Technology.
Yet the Machine-Cult was never a single mind, and its loyalties fractured as surely as the Legions' did. When Kelbor-Hal, Fabricator-General of Mars, turned against Terra, the red planet became a battleground, and the Imperium's forge-heart was sundered in the war men came to call the Schism of Mars.