From the book jacket:
Strange wickedness is rising from a remote swamp on the magical isle of Fincayra. To save his homeland, as well as his own destiny, young Merlin must voyage to frightening places - both on the land and in himself.
Joined by Hallia, the deer-woman he has grown to love, and by his own mischievous shadow, Merlin discovers a magical mirror that can alter anyone's fate. But when he looks into it, the person he sees is the person he least expects to find.
Brimming with passion and spiritual depth, T. A. Barron's fourth book in The Lost Years of Merlin epic adds a spellbinding new dimension to the legend of Merlin, the towering figure who becomes the mentor to King Arthur and the greatest sage of his time.