C. J. Cherryh: The Chronicles of Morgaine

Morgaines mission is to seal the Gates between the worlds. Teaming up with a dishonored warrior, the great horse Siptah and her terryfying sword Changeling, she goes to war with her antagonist Chya Roh, who wants the gates kept open to gain power over the worlds. The original trilogy in one volume. 682 pages, Mandarin Paperbacks 1990.