Black Sun Series by Uki Ogasawara

Gerun Fortress, under the command of Officer Leonard de Limbourg, is attacked by the prestigious general Jemaru Jan of the Middle Eastern Empire. With no hope of winning, Leonard offers his head in exchange for the safety of any survivors left in the fortress. But Jemaru isn't into executions, and torturing prisoners of war is much more interesting. Forced to sleep with a man that is his enemy, Leonard's feelings of shame overtake his mind, but lust overtakes his body...

As the former Crusader commander Lord Francis and the king continue their manipulations of both Leonard and General Jan, it is quite apparent that they intend to drive the two lovers apart. But why is that? Is Leonard really just a plaything for General Jan as the spoils of war? Or will General Jan continue to defy convention and risk everything by defying his king in order to claim the heart and body of the knight he conquered in battle?