The Faded Sun Series 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... moreThis 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. less

Sten Duncan had saved the lives of the last two of humanity's deadliest enemies, during the takeover of their planet Kesrith. Sten therefore felt responsible for them and for their future -- if any. For though the two mri were brother and sister, they represented different power-castes of their ancient warrior-race. Niun was the last of the bred samurai. Melein, though young, was perforce the last priestess-queen. But struggle and mutual danger had sealed Sten Duncan to their loyalty... moreSten Duncan had saved the lives of the last two of humanity's deadliest enemies, during the takeover of their planet Kesrith. Sten therefore felt responsible for them and for their future -- if any. For though the two mri were brother and sister, they represented different power-castes of their ancient warrior-race. Niun was the last of the bred samurai. Melein, though young, was perforce the last priestess-queen. But struggle and mutual danger had sealed Sten Duncan to their loyalty. As their blood-brother, he would have to help them flee mankind and take the long, long evasion-route across the cosmos to a legendary lost planet which might afford the mri one more chance. less

When the Mri, a proud and noble race of warriors serving as mercenaries in exchange for a planet to call home, confront the human enemies of their employers, they encounter a method of warfare alien to their system of honor. No match for the Mri one to one, despite their similar physiology, the humans fight without honor, driving the Mri to extinction with superior numbers and firepower... moreWhen the Mri, a proud and noble race of warriors serving as mercenaries in exchange for a planet to call home, confront the human enemies of their employers, they encounter a method of warfare alien to their system of honor. No match for the Mri one to one, despite their similar physiology, the humans fight without honor, driving the Mri to extinction with superior numbers and firepower. Both the humans and the regul - former employers of the Mri whose trade disputes with humans sparked the war in which the Mri were slaughtered - see them as nothing more than professional warriors, the most dangerous killers in the galaxy. They take no prisoners, they have no fear of death, they keep the company of dangerous beasts. And yet there is a deep and powerful truth at the heart of Mri culture, hidden even to the warriors, who are the hand of contact with the outise world. True secrets of Mri culture have never been known to the outside, until one human being makes direct, personal contact. To understand their ways, he will have to become more Mri than human.... less

They were the mri - tall, secretive, bound by honor and the rigid dictates of their society. For aeons this golden-skinned, golden-eyed race had provided the universe mercenary soldiers of almost unimaginable ability. But now the mri have faced an enemy unlike any other - an enemy whose only way of war is widespread destruction. These "humans" are mass fighters, creatures of the herd, and the mri have been slaughtered like animals.Now, in the aftermath of war, the mri face extinction... moreThey were the mri - tall, secretive, bound by honor and the rigid dictates of their society. For aeons this golden-skinned, golden-eyed race had provided the universe mercenary soldiers of almost unimaginable ability. But now the mri have faced an enemy unlike any other - an enemy whose only way of war is widespread destruction. These "humans" are mass fighters, creatures of the herd, and the mri have been slaughtered like animals.Now, in the aftermath of war, the mri face extinction. It will be up to three individuals to save whatever remains of this devastated race: a warrior - one of the last survivors of his kind; a priestess of this honorable people; and a lone human - a man sworn to aid the enemy of his own kind. Can they retrace the galaxy-wide path of this nomadic race back through millennia to reclaim the ancient world which first gave them life? less