The monastic texts of the same period as Nennius' all insist on presenting this Arthur as a true brigand leader of a band of horsemen, who committed pillaging and the most diverse barbaric atrocities against religious establishments. Anyway, he is not a king and Nennius states clearly that he was only a Dux Bellorum, a "war leader" serving for the Breton kings who wanted to get rid of the Saxons. And Arthur managed to stop the Saxon advances thanks to the victory of Mont-Badon (Bath?) in c. 488. It was thus the opening of a calm period for the Bretons, a period characterised by a rebirth of the former Celtic civilisation and which didn't stop until the deadly battle of Camlann (Camelford in Cornwall?) where Arthur and his rival Medrawt (Mordred) killed each other, leaving the country as prey to the invaders.
Such was the historic Arthur, war leader and not king.
The legend has done the rest, with a lot of help from Geoffrey of Monmouth and later writers. A character as famous as Arthur, and his strange name which refers to a celtic term for the bear (artos), all this has lead to an exceptional character, true king of the world, preserver of the balance of an ideal and equal society around this famous Round Table. Purely Celtic mythological narratives cristalised around him, such as the story of Merlin, the story of Lancelot of the Lake, the story of Tristan and Yseult, and above all the huge epic the knights of the Round Table and of the Quest for the Holy Grail.
The Bretons who had been conquered both politically and militarily wished to remake their history according to their nostalgia and their hopes. And since, in the 12th century, Henry II Plantagenet on becoming King of Great Britain did not feel at ease about his legitimacy, he wanted above all to claim to be heir to an exceptional King, so he encouraged his writers at that time, or at least those that were under his sphere of influence, to write and develop all that existed in the oral tradition about this character. It is thanks to that that the Arthurian epic invaded the whole of Europe in the Middle Ages and soon conquered the whole world. Even today more and more stories -purely fictional of course- are published and the legend seems more alive then ever.