P. 635

"When the sun rises in the west and sets in the east," said Mirri Maz Duur. "When the seas go dry and mountains blow in the wind like leaves. When your womb quickens again, and you bear a living child. Then he will return, and not before."

This might be a real prophecy, or it may simply be a sort of curse to strike terror and despair into Daenerys's heart. The conditions put forward have a distinct ring of fairy-tale about them, and with Martin this could just as easily be taken to work against the likelihood of them coming true as in favour of it.