Kushiel's avatar / Jacqueline Carey.

Enhanced descriptions from Syndetics:

The land of Terre d' Ange is a place of unsurpassed beauty and grace. It is said that angels found the land and saw it was good...and the ensuing race that rose from the seed of angels and men live by one simple rule:
Love as thou wilt.
Phedre no Delaunay is a woman born with a scarlet mote in her left eye and sold into indentured servitude as a child. Her bond was purchased by a nobleman, and he was the first one to recognize who and what she is: one pricked by Kushiel's Dart, chosen to forever experience pain and pleasure as one.
Phedre's path has taken a strange and sometimes dangerous course. She has lain with princes and pirate kings, battled a wicked temptress who is still determined to win the crown at any cost, and saved two nations with her courageous actions and sacrifices. Through it all she has had the devoted swordsman Joscelin at her side, who knew from the beginning what she was. Her very nature is a torturous thing for them both, and it is a bane on their lives--but he is sworn to her and by accepting who she is, Joscelin has never violated the central precept of the angel Cassiel: to protect and serve.
But Phedre's plans will put his pledge to the test, for she has never forgotten her childhood friend Hyacinthe. She has spent ten long years searching for the key to free him from his eternal indenture to the Master of Straights, a bargain with the gods that he struck so that a nation could be saved; in doing so, he took Phedre's place as a sacrifice. She cannot forget, and she cannot forgive--herself or the gods. She is determined to seize one last hope to redeem her friend, even if it means her death.
Their search will bring Phedre and Joscelin on a dangerous path that will carry them across the world, to fabled courts and splendid vistas, to distant lands where madness reigns and souls are currency, and down a fabled river to a land forgotten by most of the world.
And to a power so mighty that none dare speak its name.
"Kushiel's Avatar "is the concluding volume in Jacqueline Carey's evocative novels about the enigmatic Phedre no Delaunay; the third in a triptych of beautifully constructed historical fantasies that combine passion and danger, great battles of the sword and soul, deep eroticism, and mystical enigmas.