Necrons

The Necrons are the warriors of the Necrontyr, a civilisation believed to have vanished some sixty million years ago, leaving little behind but ruined tomb-temples on dead worlds. They appear without warning as raiding forces of metal figures built in imitation of the human skeleton, and lay waste to outposts, colonies and whole cities. Their guns flay a target away a layer at a time; their machinery erases records and disables equipment, so a raid leaves neither witness nor evidence. A Necron struck down will often repair itself and rise again; one damaged beyond repair fades from view and vanishes.