The Weeping Angels are creatures of the ‘Abstract’. They are specifically adapted to survive almost anything. They have a ‘quantum lock’ in their DNA (a fact of their biology) that whenever they are seen by any sentient being they freeze into stone. Now you can’t kill a stone and a stone can’t kill you, but then if you turn your back, if you look away, if you blink, and oh yes it can. They are faster they you would ever believe and they are deadly, except they don’t actually kill you(sometimes). What they do is displace you in time with a touch. They put you back in time, you die before your time and they absorb all the energy (potential existence energy) from the days you would have had.
The Weeping Angels first appeared in the TV episode Blink and returned again for the two part adventure The Time of Angels/Flesh and Stone. In their return we discover that, that which holds the image of an angel becomes an angel itself, and though you must look at them do not look at the eyes because as The Book of Angels says, “The eyes are the windows of the soul but the doors beware what may enter there.” The Doctor’s companion Amy looks into the eyes for too long allowing an image of an angel to be imprinted on the visionary senses of their brain.