King Arthur was a mythical king of England. According to the myth, he was taken from his father, King Uther Pendragon, when he was a baby, by Merlin a wizard who lives life in reverse. Merlin brings Arthur to the estate of a noble to be raised thinking that the noble is his father.
When Arthur gets 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 out of the stone would be the next king, so Arthur was made king. During his mythical reign, Arthur united all of England, and form 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 is 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 plans to overthrow him, and the two meet 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. This occurred in the year 539 AD, according to Welsh chronicles. After Arthur's departure, England broke up into warring factions and was not finally reunited as a united kingdom until the time of Alfred the Great in the 9th century.