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.
Western Roman Empire
The political situation was unstable. In 383, a powerful and popular general named Magnus Maximus seized power in the West and forced Gratian's half-brother Valentinian II to flee to the East for aid; in a destructive civil war the Eastern Emperor Theodosius I restored him to power. In 392, the Frankish and pagan "magister militum" Arbogast assassinated Valentinian II and proclaimed an obscure senator named Eugenius as Emperor. In 394 the forces of the two halves of the Empire again clashed with great loss of life. Again Theodosius I won, and he briefly ruled a united Empire until his death in 395. He was the last Emperor to rule both parts of the Roman Empire before the West fragmented and collapsed.