The Faded Sun: Kesrith by C.J. Cherryh

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