Exile's Gate

Exile's Gate is a 1988 fantasy novel by American writer C. J. Cherryh. It is the fourth of four books comprising The Morgaine Stories, chronicling the deeds of Morgaine, a woman consumed by a mission of the utmost importance, and her chance-met companion, Nhi Vanye i Chya.

Plot summary
Morgaine must meet her greatest challenge: Gault, who is both human and alien, and also seeks control of the world and its Gate. She will meet the true Gatemaster, a mysterious lord with power as great as, or greater, than her own.