The Byzantine Empire was a state that evolved from the Eastern Roman Empire from about 400 AD, almost vanished from the map during the Crusades of the 13th century, and finally stopped to exist on 29. May 1453, when the empire's capital Constantinople was conquered by the Turks.
In Rise of the Tomb Raider
The Byzantine Empire plays a role in the 2015 Tomb Raider Game Rise of the Tomb Raider. It is the birth place of the Undying Prophet and many of his followers. Their descendants still speak Greek, although they were exiled from the Empire and fled to Syria and later on to Siberia where they build the fabled city of Kitezh.
Besides many Documents written in Greek, some of them dating back to the Byzantine Empire, Lara Croft can find Byzantine Coins practically everywhere in Syria and Siberia. Greek, the language of the Empire, is one of the Language Proficiencys Lara can learn.
The Prophet, after finding the Divine Source, went to Constantinople around the year 970. He gathered a following, not only from the common people but also from the nobles, e.g. Decius. When the Emperor died in 977 they where forced to leave the Empire by Trinity knights that were send by the Church in the West. During these seven years there are only small hints at the politics of the Empire that can be gathered from The Apostle documents, e.g that the Emperor did not prosecute the Prophet's followers as did his successor.