A king of Britain several centuries before Arthur. Nennius says that he was the fourth Roman emepror 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.