The Roman usurper Nepotianus, of the Constantinian dynasty, proclaims himself Roman emperor, entering Rome at the head of a group of gladiators.

Roman usurpers were individuals or groups of individuals who obtained or tried to obtain power by force and without legitimate legal authority. Usurpation was endemic during the Roman imperial era, especially from the crisis of the third century onwards, when political instability became the rule.

Nepotianus, sometimes known in English as Nepotian, was a member of the Constantinian dynasty who reigned as a short-lived usurper of the Roman Empire. He ruled the city of Rome for twenty-eight days, before being killed by his rival usurper Magnentius's general Marcellinus.

The Constantinian dynasty is an informal name for the ruling family of the Roman Empire from Constantius Chlorus to the death of Julian in 363. It is named after its most famous member, Constantine the Great, who became the sole ruler of the empire in 324. The dynasty is also called Neo-Flavian because every Constantinian emperor bore the name Flavius, similarly to the rulers of the first Flavian dynasty in the 1st century.

The Roman emperor was the ruler and monarchical head of state of the Roman Empire, starting with the granting of the title augustus to Octavian in 27 BC. The term emperor is a modern convention, and did not exist as such during the Empire. When a given Roman is described as becoming emperor in English, it generally reflects his accession as augustus, and later as basileus. Another title used was imperator, originally a military honorific, and caesar, originally a cognomen. Early emperors also used the title princeps alongside other Republican titles, notably consul and pontifex maximus.