The Sycorax are a long lived humanoid species which developed an agressive interstellar tribal empire of plunderers and esclavagists after gaining access to FTL when an alien spaceship crashed on their homeworld. Morpholigically very close to humans, they mainly differ from them by the appearance of their head, being lipless and growing a bony helmetlike structure protecting most of their head like an exoskeleton.
They move through space using asteroids which they fit with FTL engines and blackmailing other species into giving away chunks of their population in slavery. Using theblood control technique they developed, they came to Earth at christmas 2006, threatening to kill every human with A+ blood unless they were given half the population as slaves but were thwarted and their ship destroyed (using alien technology salvaged from another crash). They are known to have already being wandering through space some 2000 years ago.
Source: Doctor Who (TV: The Christmas Invasion)
