Summary
The traitor sect of the Mechanicum that swore to Horus and the Chaos Gods, unshackled forbidden knowledge from the Vaults of Moravec, and now forges daemon engines on hell-forges beyond the Eye.
Lore
The Dark Mechanicum is the traitor remnant of the ancient Mechanicum of Mars — Forge Primus — Magi, Tech-Priests, Collegia Titanica, and Knight households who pledged themselves to Warmaster Horus and, through him, to the Chaos Gods as the Great Crusade gave way to the Horus Heresy in the early M31. Where the loyalist priesthood became the Adeptus Mechanicus, the renegades pursued a different covenant: that the Machine should be wedded to the Warp, and that no proscription of the Omnissiah could bar them from the deeper dark.
The Warhammer Design Studio describes them as “a corrupted reflection of Martian sanctity” — silhouettes more “spikey, spindly, and creepy” than the angular sanctity of their loyal counterparts. They keep the cogs and the cant, but their cogs turn on bound daemons and their cant carries scrap-code.
The Schism of Mars
The proximate cause of the schism was the Vaults of Moravec — “a trove of forbidden knowledge known as the Vaults of Moravec, sealed by the Emperor’s own hand” — opened by Fabricator-General Kelbor-Hal during the Heresy. Their breach unleashed “a virulent plague of scrap-code… worming its way through every system it touched,” which spread across Mars and warped the minds of those sworn to Kelbor-Hal “into crueller, more bellicose puppets.” This was, in the studio’s words, “the true beginning of the Dark Mechanicum, as genius minds were unshackled from what remained of their morality.”
Kelbor-Hal led the resulting civil war, the Schism of Mars, in a bid to throw off the Treaty of Mars and restore the Mechanicum’s autonomy. After Horus fell at the Siege of Terra, the loyalists drove their corrupted brethren from the Red Planet; many fled into the Eye of Terror with the other traitors, and there they remain.
Hell-Forges and Daemon Engines
Where loyalist doctrine forbids the modification of the Standard Template Construct (STC) and treats the Warp as anathema to the Machine Spirit, the Dark Mechanicum does the opposite. The Warmaster, in the studio’s account, “allows the Traitor factions of the Mechanicum to explore forbidden knowledge, unlock vaults of restricted technology, and tap into proscribed power sources such as the Warp.” From this licence comes their signature heresy: “By drawing forth the exo-planar energies of the empyrean, for example, and investing their creations with the fell beings that dwell within it, the Dark Mechanicum created an entirely new and utterly forbidden canon of technology.”
The product of that canon is the daemon engine — machine bodies bound to warp-entities — and the war-titans of the corrupted Traitor Legios. “Endless swarms of Stalkers scuttled from their forges as once-proscribed technologies were warped by the renegade magi into lethal weapons of war.” Their hell-forge worlds — Xana II, Sarum, Cyclothrathe, and others — replace the cradle-craft of Mars — Forge Primus with industries that breathe the Empyrean.
Doctrine of the False Omnissiah
Where the loyalist Mechanicus venerates the Emperor as the Omnissiah made flesh, the Dark Mechanicum repudiates Him as the False Omnissiah — a usurper who fettered the priesthood with treaties and proscriptions, and who walled off the Vaults that were rightfully theirs. The truth, as their hereteks hold, is that the Machine reaches its highest sanctity only when joined to the deeper currents the Emperor sealed away. Every daemon-engine they forge is a sermon against the The Sixteen Universal Laws and against the The Quest for Knowledge as the loyalists would constrain it.