Monsters-Weeping Angels

Also known as the Lonely Assassins, these aliens could only attack their targets if you took your eyes off them. The moment anyone looks directly at them, they become immobile and powerless. But, turn away, even for a split second, and they send you back randomly into the past, feeding off the temporal energy that is left behind by your life no longer existing in your proper timeline.

The Weeping Angels were winged humanoids who evolved numerous unique survival mechanisms throughout their time, including the ability to move creatures back through time with a touch. This allowed them to consume the potential energy from the time the victims could have had alive had the Angels not transported the victim back in time.
The Doctor described them as being "the only psychopaths in the universe to kill you nicely", and apparently it was rare for them not to use this method of murder. The Weeping Angels evolved near the beginning of the universe, and were (as the Doctor put it) the kindest of killers and psychopaths since their method of killing was to let their victims "live to death". This perhaps make them the loneliest beings in existence as, due to their defence mechanism, they could not even look at each other. In that respect the Doctor appears to pity them.

Episodes (as of May 2010):
Blink
A Ghost Story For Christmas
Flesh and Stone
The Time of Angels

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