Summary
The treason of Fabricator-General Kelbor-Hal, who pledged Mars to Warmaster Horus and unsealed the Vaults of Moravec, igniting the Schism of Mars and giving birth to the Dark Mechanicum.
Lore
The Heresy of Kelbor-Hal is the treason of the Fabricator-General of Mars — Forge Primus during the opening of the Horus Heresy in the early 31st Millennium. Kelbor-Hal, supreme magos of the ancient Mechanicum and de facto ruler of the Red Planet, pledged the holy world of the Omnissiah to Warmaster Horus Lupercal. His pact lit the civil war upon Mars known as the Schism of Mars, and from its embers rose the Dark Mechanicum.
Two Terran centuries earlier, the Emperor had walked the soil of Mars and bound the Mechanicum to His Imperium under the Treaty of Mars. Kelbor-Hal had bent the knee, but the equality promised never came. A faction led by the Fabricator-General held the Emperor a false idol and the manifestation of the Omnissiah yet to arrive — a doctrinal fracture decades old when Horus turned.
The Pact and the Vaults of Moravec
Horus dispatched the tech-priest emissary Regulus to Mars. He bore three offerings: autonomy for the Mechanicum, the Standard Template Construct seized from the conquered Auretian Technocracy, and — most damning — the security protocols to open the Vaults of Moravec, the depository of forbidden archeotech sealed by the Emperor’s own hand.
The Emperor had once walked Kelbor-Hal through those tunnels, then psychically wiped the Fabricator-General’s cartographic memory buffers so that he would know of the trove but never find it. Regulus restored the path. Kelbor-Hal opened the Vaults. The opening of the Vaults of Moravec marked the true birth of the Dark Mechanicum and, in the long aftermath, the present-day Adeptus Mechanicus.
The Schism of Mars
Civil war engulfed the Red Planet. The Schism drew in every order of Martian power — menial through Knight and Titan, Magos through Fabricator — and pulled in Space Marines and Imperial Army units besides. Kelbor-Hal’s traitor Mechanicum unleashed the dark and forbidden knowledge of the Vaults against the loyalist forges, and emerged victorious. Mars fell. The Dark Mechanicum stood revealed at Istvaan V, where it deployed proscribed weapons against the loyalist Astartes.
Doctrinal weight
The Heresy of Kelbor-Hal is the founding wound of the modern Cult Mechanicus. It teaches the priesthood three lessons it has not since unlearned: that forbidden knowledge is forbidden for a reason, that the Quest must be pursued under doctrine, not against it, and that a Fabricator-General is not above the Omnissiah. The schism is why the Adeptus Mechanicus of M41 still names its renegade kin not as a rival cult but as the eternal foe — the Dark Mechanicum.
The events were chronicled in the Black Library novel Mechanicum by Graham McNeill (Horus Heresy, Vol. 9).