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
In addition to remaining as a concept for an administrative unit in the remaining Empire, the ideal of the Roman Empire as a mighty Christian Empire with a single ruler further continued to appeal to many powerful rulers in western Europe. With the papal coronation of Charlemagne as "Emperor of the Romans" in 800 AD, his realm was explicitly proclaimed as a restoration of the Roman Empire in Western Europe under the concept of "translatio imperii". Though the Carolingian Empire collapsed in 888 and Berengar, the last "Emperor" claiming succession from Charlemagne, died in 924, the concept of a papacy- and Germanic-based Roman Empire in the West would resurface in the form of the Holy Roman Empire in 962. The Holy Roman Emperors would uphold the notion that they had inherited the supreme power and prestige of the Roman Emperors of old until the downfall of the Holy Roman Empire in 1806.