Kushiel's Mercy (Imriel's Trilogy #3)

There's only one way they can truly be together. Imriel must perform an act of faith: search the world for his infamous mother and bring her back to Terre d'Ange to be executed for treason.
Facing a terrible choice, Imriel and Sidonie prepare ruefully for another long separation. But when a dark foreign force casts a shadow over Terre d'Ange and all the surrounding countries, their world is turned upside down, alliances of the unlikeliest kind are made, and Imriel and Sidonie learn that the god Elua always puts hearts together apurpose.