DOCTOR: There was a goblin, or a trickster, or a warrior. A nameless, terrible thing, soaked in the blood of a billion galaxies. The most feared being in all the cosmos. And nothing could stop it, or hold it, or reason with it. One day it would just drop out of the sky and tear down your world.

AMY: How did it end up in there?

DOCTOR: You know fairy tales. A good wizard tricked it.

RIVER: I hate good wizards in fairy tales. They always turn out to be him.