Roman emperor
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the West after his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
Roman Empire
Diocletian divided the empire into four regions, each ruled by a separate emperor, the Tetrarchy. Confident that he fixed the disorders that were plaguing Rome, he abdicated along with his co-emperor, and the Tetrarchy soon collapsed. Order was eventually restored by Constantine the Great, who became the first emperor to convert to Christianity, and who established Constantinople as the new capital of the eastern empire. During the decades of the Constantinian and Valentinian dynasties, the empire was divided along an east–west axis, with dual power centres in Constantinople and Rome. The reign of Julian, who under the influence of his adviser Mardonius attempted to restore Classical Roman and Hellenistic religion, only briefly interrupted the succession of Christian emperors. Theodosius I, the last emperor to rule over both East and West, died in 395 AD after making Christianity the official religion of the empire.