That's a question we can't answer with certainty, but we can guess that Arthur is a blend of Fact and Fantasy. According to Geoffrey Asche, the historical figure from whom Arthurian legend sprang may have been a fifth century warrior king called Riothamus.
Written references to Arthurian-type figures begin with the work of Gildas, a sixth-century monk who wrote De Excidio Britanniae ("Concerning the Ruin of Britain"). Gildas' lurid description of battle against the Saxons culminates with the British "remnants" rallying behind a man called Ambrosius Aurelianus.
Nennius, a Welsh monk who wrote around the year 800, was the first to refer to Arthur by that now-familiar name. In his Historia Brittonum, Nennius lists Arthur's twelve great victories over the Saxons, finishing with the triumph at Mount Badon.
Mordred's role in Arthurian legend may also begin with Nennius, who mentions a son of Arthur, killed by his father.
By the twelfth century, the legend of King Arthur had become very popular. William of Malmesbury wrote in Gesta Regum Anglorum of Arthur's prowess as a warrior. This is a dressed-up version of the tales of Nennius and Gildas with one vital variation: in this tale, Arthur's role appears to be not that of overlord, but of general.
Geoffrey of Monmouth's Historia Regum Britanniae appeared about the same time as Malmesbury's work. Geoffrey claimed his story came from an ancient book handed to him by the Archdeacon of Oxford. The existence of such a book cannot be disproved, but it is entirely likely that Geoffrey's fantastic story came from a compilation of contemporary romantic tales and his own inventive imagination.
A number of Welsh manuscripts dated to this period also deal with Arthurian legend. (The stories themselves, however, may be even older.) Culhwch and Olwen details the exploits of Arthur and his company, while the Triads of the Isle of Britain lend us a tantalizing glimpse of stories lost to us.
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