Justinian was one of the most important rulers from the over 1000 years of rule of the Byzantine Empire. He created many codes, or laws, to unify the empire. He spread religion throughout the Byzantines. Expanding the empire was also a big achievement made by him. The most important architectural monument of the Byzantines, the Hagia Sophia, was also created at this time.
(Sorry, I'm a World History geek, I took AP World History freshman year ;))