The Byzantine emperor known for his legal reforms and the construction of the Hagia Sophia was Justinian I, who reigned from 527 to 565 CE. His reign is marked by the codification of Roman law into the Corpus Juris Civilis, which laid the groundwork for modern legal systems. The Hagia Sophia, constructed during his reign, became an architectural marvel and a symbol of Byzantine culture. Justinian's efforts sought to restore the greatness of the Roman Empire, influencing both Eastern and Western governance and culture.