Harry Turtledove 
Krispos the Emperor 

Krispos had seized the throne of Videssos as scarcely more than a peasant youth. With wit, luck, and magic, he had held it through a generation of turmoil. But now came his greatest challenge, as he struggled to save his empire from tearing itself asunder. For a strange heresy had taken root in the land, spreading secretly among peasants and zealots, among the poor and among the traitors. And all too soon, hidden dissent flared into open revolt. When Krispos led his legions against the rebels, his three sons rode at his side. Phostis, eldest of the princes, was expected to follow Krispos to the throne. But it was Phostis who had given the heretics a hearing, and now it was Phostis who disappeared into the rebel ranks. As the renegades seized their day, Krispos waged ever more desperate war against and implacable foe that would not scruple to set brother against brother, father against son…