Humanoids with an endo- and exoskeleton who controlled a third of the Earth's population through blood control and threatened to kill them unless the Earth surrendered to being sold into slavery. The Sycorax leader cut off the Tenth Doctor's right hand in a sword fight. But he was only 15 hours into his regeneration cycle so it grew back. Yep. Appeared in the 2005 Christmas special, The Christmas Invasion.