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.
Ancient Rome
In the late 4th and 5th centuries the Western Empire entered a critical stage which terminated with the fall of the Western Roman Empire. Under the last emperors of the Constantinian dynasty and the Valentinian dynasty, Rome lost decisive battles against the Sasanian Empire and Germanic barbarians: in 363, emperor Julian the Apostate was killed in the Battle of Samarra, against the Persians and the Battle of Adrianople cost the life of emperor Valens (364–378); the victorious Goths were never expelled from the Empire nor assimilated. The next emperor, Theodosius I (379–395), gave even more force to the Christian faith, and after his death, the Empire was divided into the Eastern Roman Empire, ruled by Arcadius and the Western Roman Empire, commanded by Honorius, both of which were Theodosius' sons.