Arthur defended his kingdom from a revolt by twelve lesser kings. After consolidating his power in Britain, he sailed to continental Europe to visit a cousin, who was a French noble. The people there were being terrorized by a giant, which Arthur slayed. He then took his army farther into Europe, and fought Lucius, Emporer of Rome. After defeating Lucius, Arthur was crowned Emporer.
Many later writers have incorporated Arthur’s battle against the twelve rebellious lords into their work, but few have used the tale of Arthur’s battle against Rome. Perhaps this is due to the fact that fighting against a Roman emporer would place a definitive time stamp on Arthur’s reign, and most authors wish the time period to be a bit fuzzy, so that the fantasy does not conflict with real world circumstances. Perhaps authors have been hesitant to incorporate Arthur’s fight against Rome because no such battle exists, and this might shatter the illusion of realism.