The Faded Sun
From the back cover:

This is the story of three people: Sten Duncan, a soldier of humanity; Niun, last warrior of the **mri**, humanity's enemies; Melein, priestess-queen of the final fallen **mri** stronghold.

This is the story of two mighty species fighting for a galaxy: humanity driving out from Earth, and the enigmatic regul struggling to hold their stars with **mri** mercenaries.

This is a story of diplomacy and warfare, of conspiracy and betrayal, and of three flesh-and-blood people who found themselves thrown together in a life-and-death alliance.
American Fantasy fiction, Science fiction, Fiction, science fiction, general, Fiction, general
C. J. Cherryh
Carolyn Janice Cherry (born September 1, 1942), better known by the pen name C. J. Cherryh, is an American writer of speculative fiction.