Diadem #5: Star Hunters by Jo Clayton

This vibrant heroine and the powers of the Diadem. For on the world to which it conveyed her, she must confront not only the hordes of half-humans who are devastating the planet but meet head-on the mental force of a madman of her own ancestral race. It is an encounter on several levels – human, inhuman, and superhuman – and she must triumph on all or lose everything.