The Tower of Bones (The Three Powers, Book 2)

The four friends, Alan, Kate, Mark and Mo have entered the enchanted but war-ravaged world of Tir, bringing hope for the oppressed peoples who live there. But Kate has been kidnapped by the Great Witch, Olc, who has imprisoned her in the Tower of Bones. Mark is lost in Dromenon, uncertain if he still lives, and Mo is still recovering from the injuries she suffered at the battle of Ossierel.
Alan sets out to rescue Kate, but his every move is harassed by the Tyrant of the Watelands. Olc is luring the friends into a deadly trap, for she is resurrecting the demigod, Fangorath, an immortal and and dreadful force for malice, who is said to have ended the Age of Dragons.