The Mirror of Merlin

Blurb

The Mirror of Merlin is a 1999 fantasy novel by T. A. Barron published by Penguin. It is the fourth of The Lost Years of Merlin, a five-book series providing a childhood story for the legendary Merlin, wizard of Arthurian legend.
In a remote swamp on the magical isle of Fincayra, uncommon wickedness arises. Young Merlin must journey to terrifying places - both within himself as well as on land - to save both his homeland, and his own destiny.
Accompanied by the deer-woman he has come to love, Hallia, and his own roguish shadow, Merlin discovers "a magical mirror that can alter anyone's fate." The person Merlin beholds upon gazing into the mirror is the person he least expects to find.