King Arthur was a mythical king of Britain. The myth has a number of formulations, some more dominant in the popular mind.
T.H. White's formulation, The Once & Future King
Merlin brings Arthur to the estate of a noble to be raised thinking that the noble is his father. When Arthur grows older, he becomes a page for the noble's son, Sir Kay. During this time Uther Pendragon becomes ill and soon dies. So Merlin says that they should have a grand tournament, and afterwards he will announce the next king. At the tournament Kay breaks his sword, so he sends Arthur to find him another. In his haste Arthur grabs a sword that is sticking out of a stone in the town square. As it turns out, Merlin had laid a spell on the stone that said that whoever could pull the sword - Excalibur - out of the stone would be the next king, so Arthur was made king. During his mythical reign, Arthur united all of England, and formed a league of knights called the Knights of the Round Table. When they gathered together, they all sat at a round table so no seat was better than another.
Arthur later sent his knights on a quest to find the Holy Grail, which they finally found on the Isle of Avalon guarded by an order of virgin priestesses led by Morgan la Fay. After this successful quest, Arthur's illegitimate nephew Mordred planned to overthrow him, and the two met in battle at Camlann. Mordred is killed, and Arthur, mortally wounded, is taken to the Isle of Avalon for healing by Morgan la Fay. According to legend he will only return when his country truly needs him.
King Arthur in History
The death of Arthur occurred in the year 539 AD, according to Welsh chronicles. After Arthur's departure, Britain broke up into warring factions and was not again a united kingdom until the time of Alfred the Great in the 9th century. A Celtic king named Arthur may very well have existed among the Welsh or Britons, but the legend is so obscured by mythical satire, it is at this time indeterminable.
Relation to Mono-Myth
Joseph Campbell, in his book The Hero with a Thousand Faces, describes elements of popular mythology evident in many cultures, and many different stories. One such characteristic is the obscure nature of the hero, which most iterations of the Arthur myth do satisfy.
- Annales Cambriae, which cryptically states only that Arthur and Mordred both fell at the Battle of Camlaan, with no reference to either in the rest of the text.