The Wraith are a vampiric hive-based species that harvest the 'life-force' of other humanoid beings for nourishment through suckers on their palms. Countless worlds in the Pegasus galaxy live in constant fear of the Wraith, who return periodically to cull their human herds.
"The Wraith allow our kind to grow in numbers, and when that number reaches a certain point they return to cull their human herd. Sometimes a few hundred years will pass before they awaken again. We've visited many, many worlds -- I know of none untouched by the Wraith."