The Sun at Twilight by N. L. Holmes

In the twilight years of the Hittite Empire (1230s BC), Tudhaliya IV comes to the throne as a usurper's younger son, determined to rule according to the traditional values of justice and clemency, despite the harder advice of his ambitious mother and his questionably loyal older brothers. When his cousin Kurunta, with whom he was raised as someone dearer than a brother, decides his own claim to the throne is stronger than Tudhaliya's, the latter must decide whether to apply the brutal pragmatism of his advisors or risk appearing weak, and how to manage to leave his own son an empire that is finally free from civil war. The cost will be the sacrifice of his deepest friendship.