The Changewinds — Jack L. Chalker

When the Changewinds blow across the worlds that they themselves created, they are at once random and consisten, obeying their own spectral meterology. Over this tumultuous sea of realities rule the Ahkbreed, whose sorcerers have mastery over physical laws, giving them absolute power over an empire of worlds beyond counting. And now two young women from Earth have been caught up in the fury of the Changewinds