The Overseers were a race of sentient machines encountered on the world of Dahinta by the Luna Wolves Space Marine Legion's 63rd Expeditionary Fleet during the Great Crusade in the late 30th Millennium. The Luna Wolves would go on to fight 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 the Astartes, the thinking machines were ultimately wiped from the face of Dahinta by the savagery of the Space Marines.
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 Council of Terra at the direction of the Emperor and the Adeptus Mechanicus. The Overseers, commanded by a senior machine intelligence called the Archdroid, inhabited a series of derelict, crumbling and long-empty human cities on the world of Dahinta. These were cities decorated with fine mosaics that 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 were eventually destroyed after a long and brutal campaign in which the technical skills of the Mechanicus had proven invaluable. Only then was the sad secret of this machine race's origins discovered -- the Overseers were the product of human ingenuity. Made thousands of Terran years before, perhaps even during the Age of Technology, Dahinta had once been a human colony, home to a lost branch of Humanity, where they had raised a great and marvellous culture of magnificent cities, making use of thinking machines to serve them. At some time, and in a manner unknown to the Imperials, the humans of Dahinta had become extinct. They had left behind their ancient cities, empty but for the deathless guardians they had made.
- Horus Rising (Novel) by Dan Abnett, pp. 148-149