What empire split into two with the eastern portion becoming the byzantine empire?

The correct answer for this is Roman Empire.

Explanation:

The Roman empire is divided after the death of Constantine. In 364, Emperor Valentinian I divided the empire into western and eastern sections, putting himself in power in the west and his brother Valens in the east.

The western region remained under attack and the empire in divided into numerous states where as the eastern portion became the Byzantine empire.