Weeping Angel

The Weeping Angels were a species of winged humanoids from the early universe, so called of their weeping appearance to prevent trapping themselves in stone form for eternity (see below). (DW: Blink)

Biology
The Weeping Angels were winged humanoids with sharp teeth who evolved numerous unique survival mechanisms thoughout their time, including the ability to move creatures back through time with a touch, allowing them to consume the potential energy from the time the victims could have had alive had the Angels not transported the victims back in time.

They also had a unique and nearly-perfect defence mechanism, quantum entanglement, which caused them to turn into stone when being observed. When not being observed, they could move incredibly fast to catch their victims. However, this meant that they had to cover their eyes when in their stony form, otherwise if they saw each other they would be trapped forever. (DW: Blink)

History
The Weeping Angels evolved near the beginning of the universe, and were the kindest of killers and psychopaths but also as perhaps one of the loneliest beings in existence.

In 2007, a quartet of Weeping Angels managed to strand the Doctor in 1969 and captured his TARDIS with the goal of using the Time Lord technology for its near-unlimited temporal energy, in a process which is capable of destroying a star. Despite gaining the TARDIS the Angels could not find a way into the police box-shaped construct, however they later found the TARDIS's key. Unfortunately for the Angels, Sally Sparrow gained the key from one of the Angels before they could reach the TARDIS by looking at them, causing the quantum entanglement effect to kick in, leaving a key on one of the Angels for Sally to take.

They then started stalking Sally in their attempt to gain access to the TARDIS. They managed to corner her in the basement of the building where the TARDIS was stored by the Angels, at which point she managed to enter the TARDIS and which dematerialised as the Angels tried to stop it by holding onto it. When it finished dematerialising the Angels gazed at each other across the emty sapce where the TARDIS was, which turned each other into stone perpetually, thereby ending the group's menace. (DW: Blink)