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 usurper
Commodus, the last emperor of the Antonine dynasty, remembered by contemporaneous chronicles as an unpopular ruler notorious for his extravagance and cruelty, was assassinated in 192. Without sons to be his heir, a struggle for power immediately broke out amongst the governors of the most important provinces. Pertinax was elevated to the purple and recognized by his peers, but following his murder by a restive Praetorian Guard, Septimius Severus decided to make his bid for power and usurped the throne. Although initially a usurper, Severus managed to remain in power for the next 18 years and died a natural death while campaigning in northern Britain. The death of Severus Alexander (the last emperor of the Severan dynasty) in 235 triggered what historians call the crisis of the third century. In this period, from 235 to the accession of Diocletian and the establishment of the Tetrarchy in 286, Rome saw 28 emperors of whom only two had a natural death (from the plague). However, there were also 38 usurpers who raised revolts across the Empire, a clear sign that the security of the frontiers was not the only problem within the Roman world. Usurpation attempts were a constant worry for the emperors in this period; it was a too common method of acceding the throne. Successful usurpers were usually either provincial governors, commanders of a large grouping of Roman legions, or prefects of the Praetorian Guard, which had control of Rome, where the Imperial palace still lay.