Lord of Emperors - Guy Gavriel Kay

Beckoned by the Emperor Valerius, renowned mosaicist Crispin arrives in the fabled city of Sarantium, where he seeks to fulfill his artistic destiny high upon the dome that will become the emperor's magnificent sanctuary. But Crispin is not protected from intrigue. Rumors of war and conspiracy swirl about the city, and Valerius looks west to Crispin's homeland to reunite an Empire.