Reapers

"There's been an accident in time. A wound in time. They're like bacteria, taking advantage"

Before the Time War, any tangles in time were moderated, controlled and fixed by the Time Lords. But in a universe without them, this process became more organic and terrifying. Enter the Reapers: voracious winged creatures who feed on the power of time. Drawn to paradoxes, they sterilise wounds in time by consuming everything inside.

When Rose crossed her own timeline to save her father, she created a paradox, causing the Reapers to arrive in London, 1987. The Doctor discovered that old things have a stronger resistance to the Reapers, but when he too was devoured, the only thing that could save those trapped in the paradox was the sacrifice of a hero.