Spirits of Flux and Anchor — Jack L. Chalker

Cassie did not feel the soul rider enter her body...but suddenly she knew that Anchor was corrupt, and that, far from being a formless void from which could issue only mutant changelings and evil wizards, Flux was the source of Anchor's very existence. The price of her new knowledge is exile, yet Cassie and the Rider of her soul are the only hope for the redemption of both Flux and Anchor.