Silver on the Tree by Susan Cooper
The great battle catches up Will, his ageless master Merriman, and the strange albino Welsh boy Bran, whose destiny ties him to the light. Drawn into the conflict also are the three Drew children, who are mortal but who have their own vital part to play in the story. In a quest that takes them through time and space these six fight fear and death in the darkly brooding mountains of Wales. There Will and Bran are caught into the haunting, timeless Lost Land, to find dream and nightmare - and to ultimately vanquish the Dark.