The Eternal Flame — T.A. Barron

Avalon, the great tree world connecting the earth and the heavens, is about to be destroyed. The warlord Rhita Gawr is bent on conquest--and using an army of deathless warriors, a corrupted crystal, and a plague of arrogance and greed to succeed.

Three unlikely heroes are Avalon's only hope. Tamwyn, the wilderness guide, must travel the secret pathway to the stars. Elli, the brave young priestess, must defeat a terrible sorcerer in a realm of utter darkness. And Scree, the eagleman, must lead his winged people to do what seems impossible . . .