Overseers

The Overseers were a machine race of artificial sentients encountered on the world of Dahinta by the Luna Wolves Space Marine Legion's 63rd Expeditionary Fleet during the Great Crusade. The Luna Wolves would go on to fight side-by-side with the Emperor's Children in a brutal genocidal campaign against this machine race. One of the Emperor's longest-standing commands was that no sentient machines could be allowed to exist, for they had nearly caused the extinction of Mankind when they had rebelled against their human masters during the Dark Age of Technology. Despite heavy casualties amongst both Legions, the thinking machines were wiped from the face of Dahinta by the savagery of the Astartes.

History
The Overseers were artificial sentients first encountered during the Great Crusade on the world of Dahinta by the 63rd Expeditionary Fleet under the command of then-Commander Horus and his XVI Legion. This race were quite beyond the limits of Imperial law, for machine life untempered by organic components had long been outlawed by both the Imperial Council and the Mechanicum. The overseers, commanded by a senior machine called the Archdroid, inhabited a series of derelict, crumbling cities on the world of Dahinta. These were cities of fine mosaics, which had once been very beautiful indeed, but extreme age and decay had faded them. The overseers scuttled amongst the mouldering piles, fighting a losing battle of repair and refurbishment in a single-minded obsession to keep the neglected cities intact.

The machines had eventually been destroyed after a lasting and brutal war in which the skills of the Mechanicum had proved invaluable. Only then was the sad secret found -- the overseers were the product of human ingenuity. Made thousands of years ago, perhaps even during the last Age of Technology, Dahinta had been a human colony, home to a lost branch of Humanity, where they had raised a great and marvellous culture of magnificent cities, with thinking machines to serve them. At some time, and in a manner unknown to the Imperials, the humans had become extinct. They left behind their ancient cities, empty but for the deathless guardians they had made.