Tiberius II Constantine became Byzantine emperor upon the death of Justin II in 578. Before that he had been Caesar, co-emperor to Justin, and in actual power together with Sophia, the emperor's wife. The two of them had declared Justin II as mentally unfit to reign, and taken over control of the government.
After Justin II's death, Tiberius became sole ruler of the Byzantine Empire. Empress Sophia tried to keep her power and influence by marrying the new emperor, but Tiberius refused the marriage proposal.