Leo, or Leon (Λέων) I was a Byzantine and Eastern Roman emperor of the 5th century. He was the founder of the Leonid dynasty and is known for being the first emperor to legislate in Koine Greek. His reign also included his interactions with the Ostrogoths with Theoderic the Great and the Vandals and his attempts to reclaim lost territory, but ultimately the Balkans were seized by the Ostrogoths.