The Holy Empire of Man, in its quest to conquer the galaxy used every means available.  One of the more disturbing of these was the creation of Revenants.  When the forces of the Empire would find one of the Emperor's enemies on the battlefield, either brain-dead, or severely brain-damaged, but otherwise alive, Imperial surgeons would often remove the casualty's head, and replace it with a neuro-cybernetic controller, and some basic sensors.  Then, they would release the Revenant, either guided by artificial intelligence, or under remote-control, as a psychological warfare agent.

See:  Tail of the Toad God