ICO: Castle in the Mist (novel)

When a boy named Ico grows long curved horns overnight, his fate has been sealed—he is to be sacrificed in the Castle in the Mist. But in the castle, Ico meets a young girl named Yorda imprisoned in its halls. Alone they will die, but together Ico and Yorda might just be able to defy their destinies and escape the magic of the castle.