Three Big Things:
1. He remains in many ways a convenient personification of the Byzantine Empire (which lasted over a thousand years) and came closer than any other ruler to rebuilding the Roman Empire, both in terms of geographical reach and monumental architecture like the Hagia Sophia – a massive, beautiful Greek Orthodox church which later became a mosque and is now a museum and one of Turkey’s most-visited tourist destinations.
2. He compiled, edited, and rewrote centuries of Roman statutes and case law to bring order and predictability to the courts and law schools. “Justinian’s Code” stabilized and standardized law across the empire and shaped the future of European jurisprudence.
3. He married Theodora, a former child actress and prostitute. Together they were reform-minded and brutal, progressive and vicious. Whatever their naughty-to-nice ratios, they certainly accomplished plenty of stuff.