Possessing a natural and unique defence mechanism, the Angels are quantum locked. This means that they can only move when no other living creature, including their own kind, is looking at them. As soon as they are observed, they instantly turn to stone and cannot be killed.
The Angels have the ability to send other beings into the past. This allows the Angels to feed on the potential time energy of what would have been the rest of their victim's lives.
The only psychopaths in the universe to kill you nicely.The Doctor
In The Time of Angels the Doctor and Amy along with River Song went hunting for a solitary Angel but discovered an army of them waiting in the Maze of the Dead! Although the Lonely Assassins vastly outnumbered their prey the Doctor was able to outwit them using gravity and the army was defeated. The Time Lord next confronted them in New York, 1938, where another terrifying battle was fought. Although Rory was able to escape the grisly fate they had intended for him, involving years spent in Winter Quay without Amy, this encounter had a heart-breaking end when the Ponds were both taken from the Doctor – zapped back in time by a solitary Angel in a graveyard.