In OTL "King Arthur" is a semi-mythical figure who is generally accepted as having a historical basis, whether a single person or a fusion of several historical characters. One possibility is that Arthur was a Roman centurion or standard bearer (the name has a connection with a word for 'bear' and standard bearers wore bearskins as part of their uniform), and his 'knights' were the (ex)legionnaries associated with him.
His story has been developed and revived at various points (the Norman period, the Welsh, Thomas Malory, Henry VII's eldest son Arthur, the Victorians and the modern period). In each period the tale reflecting current needs and interests (chivalry, the Grail and so on), thus forming althistories of each other and what actually happened.
In this timeline Arthur becomes a more recognisably historical figure, being the founder of a successor state to the Romans, based on Glastonbury.
The Annals of Cambria note that in 539 AD: The Battle of Camlann, in which Arthur and Medraut both fell; and there was a widespread death in Britain and Ireland. Tradition has Arthur and Mordred (as he is more generally known) on opposite sides: however they could equally well have been on the same side, as is the case in this timeline.