You might be referring to Justinian I, or The Great, who ruled as emperor of the Byzantine Empire from 527 to 565. Justinian I wanted to reconquer the western half of the Roman Empire, and was partly successful. Justinian’s generals reconquered the Vandals in North Africa, regained Rome and some parts of the Italy from the Ostrogoths, and Spain. His reign is marked by numerous reforms, and helped rewrite the Roman Law that is the basis of civil laws in many countries today. So the guy cannot be called a tyrant. He is, however, a hero to the Christians.