Shadow of the Well of Souls — Jack L. Chalker

Immortal, unchanging, the eternal survivor, Nathan Brazil had tired of his long duty as the guardian of the Well World and had enlisted Mavra Chang, space pilot and adventurer, as his companion and equal, sharing with her the godlike power to control the universe's destiny. Now the Well World is changing, and Brazil and Chang have drifted apart . . . but they must work together one more time to stop a new threat to the Well World -- the greatest threat of all . . . .