Weeping Angels

"Fascinating race, the Weeping Angels. The only psychopaths in the universe to kill you nicely. No mess, no fuss, they just zap you into the past and let you live to death. The rest of your life used up and blown away in the blink of an eye. You die in the past, and in the present they consume the energy of all the days you might have had, all your stolen moments. They're creatures of the abstract. They live off potential energy."

- The Doctor describing the Weeping Angels.

The Weeping Angels are an ancient race of alien monsters in Doctor Who, appearing as recurring enemies of both the Tenth and Eleventh Doctors. They first appeared in the Third Series episode "Blink".

Background
The Weeping Angels are a ancient race that are described by the Doctor as "the deadliest, most powerful, most malevolent life-form evolution has ever produced" as they can sometimes kill without a thought. However, the Weeping Angels prefer to send a victim back in time before they were born to feed on the temporal energy the represents the life that person would had live through. As a reflex defense mechanism, though it renders them unable to socialize with each other, the Weeping Angels appear as statues when looked at by another. However, once their target's gaze is off them, the Weeping Angels can move upon them in less than seconds' time. Even the image of a Weeping Angel that take a life of its own.

History
The Weeping Angels chronologically first encounter the Doctor when they, in connection with Julius Grayle, took over 1920s Manhattan and made it into their feeding ground. Even the Statue of Liberty was one of them. Though the Eleventh Doctor managed to negate their presence in that time, he lost his companions Amy Pond and Rory Williams in the process.

In their first appearance, a group of Weeping Angels in the abandoned Westar Drumlins mansion trapped the Tenth Doctor  and his companion Martha Jones in the past and took his TARDIS to nourish on it energy. However, via a time paradox, the Doctor enlists the aid of a woman named Sally Sparrow to sent the TARDIS back to him while tricking the Weeping Angels into looking at each other to freeze them indefinitely.

At some point in time, at the prison ship of the Minotaur the Doctor encountered fake Weeping Angels, which were crested to impersonate the biggest fear of the person to enter their room.

In the distant future, most of the Weeping Angels have occupied Alfava Metraxis but ended up in a state of suspended animation as they wiped out the native race prior to human settlers arriving. However, a Weeping Angel trapped in the vault of the ship Byzantium has the vessel crash into Alfava Metraxis to awaken its kind from their slumber. The Eleventh Doctor, accompanied by Amy Pond, manages to wipe out all the Weeping Angels by having them get sucked into a crack in time which erased them from existence.