Kesrith (The Faded Sun, Book 1)
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.