The Schism of Mars

When the Warmaster raised his banner of rebellion, his corruption reached even to the Red Planet, the ancient heart of the Adeptus Mechanicus. The Schism of Mars saw the sacred forge-world torn apart from within as the Fabricator-General Kelbor-Hal, long resentful of the Emperor's restrictions upon forbidden knowledge, declared for Horus and unleashed the horrors of the Dark Mechanicum.

The civil war that followed was a nightmare of cog and flesh. Forge fought forge, and the skitarii legions turned their weapons upon one another across the rust-red plains and within the cavernous manufactoria. Loyalist magi who refused to abandon their oaths to Terra were hunted through the pipe-choked depths, while scrapcode and daemon-tainted machines corrupted all they touched. The very air of Mars, thick with the smog of ten thousand years of industry, grew heavy with betrayal.

Though pockets of loyalist resistance endured, Kelbor-Hal's traitors seized the greater forges, and their monstrous output flowed to Horus throughout the Heresy, arming the traitor host with weapons of terrible power. The Schism did not end with the Heresy; the wound it inflicted upon the Mechanicus festered for millennia, and the servants of the Machine God never again trusted fully in the unity of their ancient priesthood.