The Weeping Angels are terrifying creatures that send you back in time if they touch you, feeding off the paradoxical energy created by your erasure from time. They freeze into stone if observed, so they can only move if you are not looking at them. However, they move at lightning speed and so even the slightest blink can be fatal if you are facing one.
The League has found no way of stopping the Angels and they continue to cause a problem. The Time Lords try and reverse their attacks but the Pantheon of Dischord aid the Angels in return for protection which makes things even more difficult.