Sundering, book 2: Godslayer
Jacqueline Carey2005

Once human but now immortal, Supreme Commander Lord Tanaros chose darkness when driven mad by the betrayal of his adulterous wife and his liege king who cuckolded him. A thousand years later, his only allegiance is to his master, the dark god Satoris. The name of Satoris is the word for evil throughout all the races, and the legend of Tanaros has become the byword for treachery. Now there is a new prophecy that tells of Satoris's destruction and the redemption of the world. To thwart it, Tanaros is holding captive the Lady of the Ellylon, the beautiful Cerelinde, to prevent her alliance with the last High King of Men. Though all who support Satoris clamour for her death, Tanaros refuses to act like the monster that he is made out to be. He discovers that not all of his heart has been lost - and his feelings for Cerelinde could doom Satoris, but also save the race of Men.