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Heart of winter
The heart of winter is a place glimpsed by Bran Stark when he is lying in a coma after his fall. Bran looks beyond the Wall, and beyond the curtain of light (possibly an Aurora Borealis) at the edge of the world, into what he calls the heart of winter. What he sees there makes him cry. The three-eyed crow tells Bran that now he knows why he must live: because winter is coming. 
Bran then sees spires of ice rising up to impale him and the bodies of a thousand dreamers before him. The crow tells Bran that he must choose: fly or die. Bran spreads his arms and flies.