The Weeping Angels were introduced to the Tenth Doctor in an episode, written by Steven Moffat, called Blink.
Also known as the Silent Assassins they throw people back in time and feast off the energy of the life that they would have lead.
The Weeping Angels, however, turn to stone if they’re being watched by any intelligent creature. This includes other Weeping Angels – and so they often cover their eyes to avoid themselves being lifelocked, stucked watching each other while in stone form.
In the Time of Angels the Eleventh Doctor meets up again with River Song as they go hunting Weeping Angel among the ruins of Alfava Metraxis. Time of Angels is the first of a two-part episode written by Steven Moffat.