Weeping Angels first made their appearance in 2007 in Blink.  They are a truly terrifying race of predatory creatures send their victims back into the past and then who feed off  the "potential energy" of the years which their victim would have lived in the present. The Doctor describes the Weeping Angels as "the only psychopaths in the universe to kill you nicely" because their victims are otherwise uninjured and may live out long and potentially fulfilling lives in the past