The famous leader known for establishing the Byzantine Empire was Emperor Constantine the Great. He reigned from 306 to 337 AD and is best remembered for founding Constantinople (modern-day Istanbul), which became the capital of the Byzantine Empire. Under his rule, he enacted significant reforms, including the establishment of Christianity as a favored religion, significantly impacting the culture and politics of the region. Constantine's efforts to unify the Roman Empire and his vision for a Christian society played a crucial role in the transition from the Roman Empire to the Byzantine Empire. His legacy endures in both religious and political history.