An Inheritance of Stars
Unlike other xenos races Reavers are, or once were human. Twisted by madness and radiation, the Reavers are packs of violent scavengers and raiders. They are known for vicious attacks on civilian ships, and the few survivors speak in haunted tones of the atrocities and horrors which occurred during these attacks.
There is no known organization or leadership within Reaver packs. However they are apparently able to cooperate well enough to crew and operate starships and also to hunt in packs. Similarly it is unknown the process by which new Reavers are created as their numbers disprove the theory of intermittent space madness.