The Dark Mechanicus, also known by its more archaic name as the Dark Mechanicum, is a sect of traitorous Tech-priests and members of the Collegia Titanica of the Adeptus Mechanicus who willingly swore their loyalty to the Warmaster Horus and the Chaos Gods as the Great Crusade came to a close and the terrible galactic conflict known as the Horus Heresy erupted in the early 31st Millennium. After the Horus Heresy ended in a pyrrhic victory for the Imperium, the traitorous Forces of Chaos that had served the Warmaster were driven towards the Eye of Terror during the brutal military campaigns remembered as the Great Scouring. The Renegade Tech-priests of the Dark Mechanicus were also driven from Mars by the resurgent Loyalists amongst the Adeptus Mechanicus. These Renegades fled the Imperium into the far corners of the galaxy and some also took refuge in the Eye of Terror. Over the centuries, their ranks have been swelled by those Hereteks of the Mechanicus who choose to follow their foul path into forbidden tech-heresy. In exile, the Dark Mechanicus became even more enthralled to the power of the Warp and heretical technologies.
The members of what became known as the Dark Mechanicus supported Horus during the start of the Horus Heresy and participated in the attacks against the Loyalist Space Marine Legions on Istvaan V where they used dark and forbidden knowledge to help destroy the Loyalist Astartes. Later, the Dark Mechanicus, led by the Fabricator-General Kelbor-Hal, unleashed the terrible civil war upon the Mechanicus known as the Schism of Mars in a bid to restore the autonomy of the Mechanicus from the Imperium of Man. This effort failed when Horus was slain at the end of the Horus Heresy during the Battle of Terra and the Loyalist elements of the Mechanicus succeeded in driving their Chaos-corrupted brethren from the Red Planet. Many of these so-called Hereteks of the Dark Mechanicus fled into the Eye of Terror alongside the other Traitors after the Great Scouring, when the Imperium recovered most of the territory across the galaxy that had been lost to Horus' Traitor forces.
At present, the Tech-priests and Dark Magi of the Dark Mechanicus, who are considered Hereteks amongst their counterparts in the Cult Mechanicus, have pledged their souls to the worship of the Dark Gods of Chaos. Within the Eye of Terror they continue to service and maintain the war machines and wargear of the Traitor Legions, the Traitor Titan Legions and the other Forces of Chaos with equal fervour. They plumb the depths of secret and forbidden knowledge kept hidden by the Ruinous Powers. Still dedicated to the acquisition of all knowledge in the universe much like their Loyalist counterparts, the Dark Mechanicus believes their uncorrupted brothers and sisters in the Adeptus Mechanicus are fools, for they will never be able to fully comprehend the divinity that is true knowledge if they cut themselves off from the secrets offered by the Dark Gods. The Tech-adepts of the Dark Mechanicus see the Omnissiah of the Cult of the Machine as being embodied in the power of Chaos Undivided rather than the Emperor of Mankind. As such, they pledge themselves to the destruction of the God-Emperor, who they believe is a false prophet who has led the rest of their fellows astray. The members of the Adeptus Mechanicus are horrified by what they view as their Chaotic counterparts' tech-heresy, and feel that the Dark Mechanicus' knowledge is a blasphemous affront to the Machine God and his Omnissiah, even as they are always curious to learn more about what their dark brethren are up to...
Kelbor-Hal was once the Fabricator-General of Mars, the political leader of the Adeptus Mechanicus and the Magos Mechanicus of the Cult Mechanicus during the latter days of the Great Crusade and the Horus Heresy in the early 31st Millennium. When the Horus Heresy began, he willingly swore allegiance to the service of the Warmaster Horus and the Forces of Chaos, becoming the first leader of the Dark Mechanicus. In the name of his new allegiance, Kelbor-Hal would unleash the terrible civil war known as the Schism of Mars upon the Red Planet, the sacred heart of the faith of the Machine God. The ultimate fate of the traitorous Fabricator-General Kelbor-Hal is not recorded in Imperial records.