The Weeping Angels (also known as the Lonely Assassins) are an ancient race of aliens from the Doctor Who television series. Steven Moffat, their creator, attributes their appeal to childhood games such as Grandmother's Footsteps and the notion that every statue is secretly a Weeping Angel. Their usual mode of feeding is to send their victims back in time, which creates time energy to feed on. When they are not being observed by another being, they can move very quickly and silently, but when they are being observed, they become 'quantum-locked', occupying a single position in space and becoming stone. In this state, they are frozen and difficult to destroy. They cannot suppress this reaction.
Contributions by Medeis, Mezigue, and DonQuixote.