Sorceress of the Witch World - Andre Norton

"I summon your banner." With these words, 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.