Marcus Aurelius Mausaeus Carausius
Carausian revolt, AD 286-296
A king of Britain several centuries before Arthur. Nennius says that he was the fourth Roman emperor to come from Rome to Britain, and that he immediately succeeded the dead Severus.
While Bassianus held the throne in Britain, according to Geoffrey’s account, Carausius convinced the Roman senate to put many ships and soldiers under his control for the purpose of defending Britain’s coast. Carausius used the soldiers, however, to overthrow Bassianus and take the throne of the island for himself. Carausius’s rule was tyrranical but brief: upon hearing of his betrayal, the Roman senate sent the war leader Allectus to rectify the situation. Allectus killed Carausius and assumed the kingship.
Both Nennius and Geoffrey are thinking of a late third-century Roman admiral who planned to usurp the Roman emperor Diocletian. Allectus, who murdered him, was his finance minister.
According to Wikipedia, Carausius came from a Belgish tribe, the Menapii, and was in time appointed a naval commander at Boulogne. His mission was to hold the English Channel clear from Frankish and Saxon raiders. Maximian, the western emperor, issued an order to kill Carausius, when words spread that Carausius collaborated with the raiders to fill his own pockets.
Carausius declared himself emperor of northern Gaul and Britain (Imperium Britanniarum) and called himself “Emperor of the North”. He held power for seven years before he was assassinated by his finance minister Allectus.
Hector Boece used Carausius to write his story about Carantius – a prince of Scotland who is suspected to be involved in his brother’s murder, is exiled and enters Roman service as a commoner. He later allied with his nephew, King Crathlinthus, against the Romans.
Historia Brittonum | Probably Nennius, early 9th century
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138