The Dark Mechanicum is the shadow cast by the Machine God's cult, a priesthood of tech-adepts who long ago traded the sterile worship of the Omnissiah for communion with the Ruinous Powers. Its members reject nothing of their old learning; they simply refuse its limits, marrying arcane engineering to daemonology until the boundary between machine-spirit and bound daemon dissolves entirely. Where the loyal cult forbids innovation, the heretek covets it, and pays for it in souls.
Much of the order traces its damnation to the great treachery of the Horus Heresy, when whole forge-worlds declared for the Warmaster and were dragged, still smoking, into the Eye of Terror. There, unbound by physical law, their manufactories became temples of blasphemous production. Rank within the Dark Mechanicum is a brutal meritocracy of knowledge, its Magi and Fabricators rising through discovery, betrayal, and the successful binding of ever-greater entities into iron.
The fruits of their labour are the terror weapons of the traitor cause. Daemon engines that scream as they hunt, possessed war-plate, and self-aware ordnance flow from their forges to the war-hosts of the Chaos Space Marines, who depend upon the hereteks utterly for the maintenance of their ancient armour. This dependence is a leash the Dark Mechanicum is careful never to pull too hard, for it profits more from being indispensable than from being feared.
Unlike the Traitor Legions, the order has no single doctrine or master. Its covens scatter across the warp-torn galaxy, each pursuing its own forbidden research, united only by a shared contempt for the timid dogma they abandoned. To them, the flesh is a poor first draft and the daemon a superior kind of firmware, and the whole material universe is merely raw stock awaiting a more inspired design.