The Scariest Monsters known to the Doctor
The Weeping Angels are an Unknown Species because nobody knows where they have come from. They are also called The Lonely Assassins.
When they first appeared they were 4 of them trying to get into the Tardis for Power. The Doctor told Sally Sparrow don't Blink as they come closer to you. They almost attacked her and her Boyfriend but they can't look at each other but they did and they could never move again.
Years Later they returned as 1 Weeping Angel made a Ship Crash and it went underground to Thousands of Weeping Angels Regenerating. When The Doctor found out the Stone People were Angels he was Shocked. 1 Angel came out a Screen and they came Alive until The Doctor and River Song stopped it. The Angels followed The Doctor and the Rest of the Team as they went one by one until The Doctor, Amy and River were the only ones Left. The Doctor Stopped them and they were destroyed all of them.
They have appeared in Series 3: Blink.
Series 5: Time of Angels and Flesh and Stone.