As the Annales Cambriae records for the year 537,

The battle of Camlann, in which Arthur and Medraut fell: and there was plague in Britain and Ireland.
This has the distinction of being the only vaguely historical record that exists for Arthur. One says vaguely because the Annales Cambriae dates from the tenth century and is therefore hardly contemporary; its dating of the battle to 537 is not necessarily correct even if one accepts it is describing an actual historical event.

In the fertile imagination of Geoffrey of Monmouth, Medraut or Medrawd becomes Mordred, Arthur's nemesis, who attempts to usurp the throne of Britain whilst Arthur is off fighting the emperor of Rome. Arthur returns to Britain to face Modred on the battlefield; Arthur kills Modred but is mortally wounded in return and is carried off to the Isle of Avallon. The Battle of Camlann therefore becomes a central feature of Arthurian Mythology which signifies the end of the Arthurian Golden Age

It is however worth pointing out that the Annales Cambriae says only that "Arthur and Medraut fell"; it says nothing about them fighting on opposite sides. They could just as well have been fighting together. Although in truth many professional historians would doubt that they ever fought at all.

Theories about the location of the battle abound, suggestions include Camelford in Cornwall,(which is convenient if you fancy that Avallon is really Glastonbury), Camroglanna now known as Birdisworld north of Hadrian's Wall, or at Cadlan on the Llyn Peninsula in Wales or even in Scotland. In reality, no one has the slightest idea, and the arguments have more to do with attracting the tourist pound (or dollar or euro) rather than historical truth.