They are the enemy that defeated the Ancients themselves long ago. The Wraith are a hive-based species with a queen, and horrifically feed off the life force of other beings. They maintain a thousand worlds populated by beings used for food. The Wraith are described as having pale green-tinged skin and black eyes, and have an incredible power to regenerate -- so much so that Dr. Beckett speculates that they may not even die of old age the same way that humans do. They also appear to have some form of psionic ability.