Krispos the Emperor — Harry Turtledove

Krispos had held the throne of Videssos since he was scarcely more than a peasant youth. But now a strange heresy has taken root in the land, a hidden dissent that is flaring into open revolt. As Krispos leads his legions with his three sons, against the rebels, one son disappears into the rebel ranks. Then the renegades seize the day, and Krispos wages an ever more desperate war against an implacable foe that would not scruple to set brother against brother, father against son....