P. C. Regalianus (died 260/261), also known as Regalian, was Roman usurper for a few months in 260 and/or 261, during the Crisis of the Third Century, a period of intense political instability in the Roman Empire. Regalianus was acclaimed emperor by the troops along the Danube river, a region of the empire that frequently experienced barbarian raids, probably in the hope that he might be able to secure the frontier.
Accounts by surviving literary sources concerning Regalianus are brief and few in number, and are mostly considered unreliable. The Historia Augusta relates that he was of Dacian descent, and a descendant of the Dacian king Decebalus, but this is mostly rejected in modern scholarship. Regalianus was married to Sulpicia Dryantilla, a woman from a prestigious senatorial family, which instead points to Regalianus also being of high-ranking Roman descent. Regalianus' acclamation as emperor was in the wake of a previous usurpation attempt by Ingenuus, also proclaimed by the Danube troops, that had been defeated by emperor Gallienus (253–268). Unlike Ingenuus, and revolutionary for an imperial claimant, Regalianus founded his own mint at Carnuntum, his seat of power. He minted coins of himself and his wife, though they were typically of poor quality.
Regalianus' local usurpation of power was beneficial for Gallienus, as it allowed the emperor to focus his attention on defending Italy from an invasion by the Alemanni while Regalianus was occupied fighting the barbarians in Illyria. After a brief "rule" of several months at Carnuntum, Regalianus was killed. How exactly he met his end is not entirely clear, but the most commonly accepted theory is that he died during a raid of Carnuntum by the Roxolani, possibly aided by a contingent of his men who had grown to oppose him.
Background
Very little of Regalianus' life and career is recorded in surviving literary sources. The often unreliable Historia Augusta, a late Roman collection of imperial biographies, records that Regalianus was of Dacian origin, and a descendant of Decebalus, the king of Dacia who had been defeated with great effort by emperor Trajan in 105–107. This story is unlikely to be true, and it is more likely that Regalianus was a Roman of senatorial rank, given that he was married to Sulpicia Dryantilla, a woman of a prestigious senatorial family. The full name of Regalianus, P. C. Regalianus, only appears on his coinage, with literary sources referring to him only as Regalianus. Some ancient historians even got this name wrong: Eutropius (late fourth century) called him 'Trebellianus' and Aurelius Victor (also late fourth century) called him 'Regillianus', 'Religilianus' and 'Religianus'. Aurelius Victor's misspellings might reflect deliberate distortion or the pronunciation of his name in Vulgar Latin.
Given the scarcity of the source material, Regalianus' full name cannot be ascertained beyond the abbreviated two first names. It is possible that the P stands for the praenomen Publius. The C might stand for the nomen Cassius, given that Regalianus could be related to the earlier suffect consul C. Cassius Regallianus, Cornelius or Claudius, but it has also been suggested to stand for a second praenomen Gaius (which was traditionally shorted archaically as "C." for "Caius" as it was originally spelled). Charles Theodore Beauvais de Preau believed based on engravings of coins by Hendrick Goltzius that his named was "Quintus Nonius Regillianus" but Joseph Hilarius von Eckhel thought that they instead belonged to some other Princeps and that the coins were the result of ignorant moneyers who failed to make legible coins, John Yonge Akerman noted that the standard of the cross described to be on the mint makes dates them to a later period, his opinion was that this supposed "Nonius" would have been a usurper during the reign of Constantius II.