Arthur as a Returning King:

Historically, the location of Arthur's grave is at best disputed and at worst non-existant. Many sites like Glastonbury have been proposed most notably by Gildas; however, in the Welsh tradition there was a translation that stated Arthur's grave was "a thing difficult to obtain" leading to the belief that Arthur was not dead and would have a Messianic return. Even the title of White's novel The Once and Future King is suggestive of a triumphant return. Malory's tradition has no subtlety in the aspect of return, stating freely that he will go to Avalon to heal himself as he departs from Britain on a black barge. Even in Geoffrey of Monmouth's The History of the Kings of Britain there is no recorded death of Arthur but the passing on of the throne as Arthur ventures to Avalon to heal.

Malory, Thomas, Le Morte d'Arthur

Wilhelm, James J. The Romance of Arthur: An Anthology of Medieval Texts in Translation / Edition 1