Weeping Angel

The Weeping Angels are one of the oldest species in the Doctor Who Universe and also one of the most malevolent. They are so named due to their trait of covering their face with their hands, giving them a seemingly weeping appearance. This is a defense mechanism to prevent trapping themselves in stone for eternity by making eye contact with another individual of their kind.

Over millions of years, the angels 'evolved' a highly effective defense mechanism, by becoming quantum locked. This means that they do not exist when being observed by any living creature, but turn to stone. As soon as the observer looks away, the angels can move. Being such a deadly species, the angels gained the epithet "the Lonely Assassins". On at least two occasions (three if the individuals in the hotel are included) they encountered the renegade Time Lord known as the Doctor.

Description
With the appearance of stone statues under observation, the Weeping Angels are an ancient race of humanoids who are known primarily for their main defense mechanism: to go into quantum lock and turn to stone, assuming the appearance of stone statues, if observed. In their stone form they appear (as their name says) as an angel, normally holding their faces with their hands. Technically, it would be impossible to see what they look like not as stone, because any image of a weeping angel would in turn become a weeping angel itself.

Physiology
Little is known of the actual biological make-up of such a creature primarily because it is theoretically impossible to examine them: examination means we could see them and to see them would trigger their ability and turn to stone. However, they are also quite fast when not observed, with the appearance of them being statues helping them survive and look harmless.

Even in their unobserved form (i.e. when moving), they continue to have the appearance of a marble-crafted object. Their senses (vision, touch, hearing) are working normally. They cannot speak human languages unless they have a human vocal cord (which they obtain by taking that of a human by force). While they consume things like energy, electricity, and radiation, they are nonetheless shown to have sharp, pointed teeth.

Their name comes from an adaptation that they have evolved. They look "weeping" as they are frequently seen covering their eyes, so as to prevent two angels from looking and freezing each other for eternity. It is thus notable that to put a mirror in front of an angel is tantamount to imprisoning it forever. Due to the widespread distribution of humans across the Doctor Who universe, their defense mechanism has proved effective.

While the Angels can kill (and sometimes dismember) humans with chilling skill, their main attack is to transport a victim into the past, feeding on the "potential energy" of the lives the people could have led, and effectively "killing" the person in the present time.

They have the same height and dimensions as that of a human, making them easily assume the appearance of a human marble statue and thus appear totally harmless.

Reproduction
According to the Book of the Weeping Angels, (a book on them that does not have any images of them) angels have a unique, parasitic, and terrifying way of reproduction: image-based reproduction - "the image of an Angel is itself an Angel."

Examples of this shown in the episode The Time of Angels, where an angel's video image, captured by a security camera and shown on a television, gains awareness on its own. Another is when Amy Pond stares at an Angel's eyes, "trapping" an angel's image in her eyes. An angel could have emerged from her and killed her, yet this "infant" angel was "paused" by shutting off the visual centre.

However, angels are not immortal, as like the stone statues they mimic so well, they are worn away by the elements, a process that is their equivalent of starvation.

History
The Weeping Angels seem to have originated at some point during the early universe. No one is sure exactly where they came from, though the Time Lords of Gallifrey appeared to know of them. On the last day of the Last Great Time War, Lord President Rassilon forced two dissident Time Lords to "stand as monuments to their shame, like the Weeping Angels of old". The Time Lords covered their faces with their hands in the manner of the angels. Some have speculated on a link between the Time Lords and the angels, though no evidence has been found to prove this link.

They have been known to attack human colony worlds and commit wholesale chrono-murder of civilizations, such as the annihilation of the Aplans in the 47th century.