Child of the Dark Prophecy — T.A. Barron

Long ago, Merlin planted a magical seed that grew into the Great Tree of Avalon. And for centuries, Avalon flourished. But now Avalon reels from brutal attacks, mysterious droughts, and strange evils. And then one night, all the stars in the sky suddenly go dark. Now the fate of Avalon rests with three young people: Tamwyn, a homeless wilderness guide; Elli, an escaped slave turned priestess; and Scree, an eagleman with a secret. One is the true heir of Merlin -- the only person who can save Avalon -- and one is fated to destroy it.