The Enslavers are among the most feared and least understood horrors in the galaxy, warp-predators that prey upon the minds and souls of the living. They are not creatures of flesh but entities of the Warp, able to force their way into reality by seizing control of a psychic host and tearing open a breach between dimensions.
An Enslaver takes possession of a psyker, warping the victim's body into a grotesque gateway through which more of its kind can pour. Those caught nearby are dominated in turn, their wills subsumed and their bodies made into puppet-thralls that serve to spread the infestation further. Left unchecked, a single Enslaver can escalate into a plague that consumes a whole world, transforming its population into a shambling host of the enslaved.
It was a great plague of Enslavers, unleashed when the barrier between reality and the Warp collapsed, that is thought to have brought the ancient empire of the Old Ones to final ruin at the close of the War in Heaven. Should such a plague ever rise again on that scale, it could threaten every sentient species at once, which is why even bitter enemies have been known to unite against them. The Enslavers remain a nightmare from the galaxy's oldest history that has never fully gone away.