Sorceress of the Witch World (Witch World #6)

Kaththea the Sorceress called forth a power such as no longer existed on the distant planet known as the Witch World. It was a power so great that it could destroy all that she loved best - and might even prove to be a greater evil than the shadow itself. Yet there could be no other choice for Kaththea than to call on Hilarion in the death-naming. For she was a witch deprived of power and she needed a guide to regain her lost skills and her lost world. There was only this ancient one, the opener of gates, with force mighty enough.