The Sycorax are an alien race that first appeared in The Christmas Invasion. The space probe, Guinevere One, crashed into a Sycorax spaceship near Mars. The Sycorax's intent was to enslave humanity, using blood control to force humanity into surrendering. Using their technology, their leader controlled everyone with the A+ blood type, commanding them to stand on the top of the tallest buildings. Harriet Jones and several people from UNIT are taken on board the Sycorax mother ship, where they deliver their ultimatum: Half of the world to be sold into slavery or a third die.
Meanwhile, Mickey, Rose and The Doctor are inside the TARDIS. The Sycorax pick up a signal from the TARDIS and transmatted them up. Eventually, The Doctor activated the blood magic but no humans killed themselves; their survival instincts were too strong. The Sycorax were forced to leave Earth and humanity alone after the Doctor defeated their leader in a sword duel. Although the Sycorax ship was leaving, the Prime Minister of the time, Harriet Jones, ordered Torchwood to fire. Five beams of green light converged over London (a la Death Star) and destroyed the spaceship outright.
Physically, the Sycorax appear to be humanoid creatures with red skin and a white exoskeleton on their heads. Though they have advanced technology, like space travel and energy whips, they are still a supernatural race who believed in magic and witchcraft.