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King Arthur was a king of Camelot, one whose exploits were the stuff of legend into the 20th and 21st centuries. It was debated among various sources weather Arthur was a real historical figure or was nothing more than a legend. According to one source, the Fifth Doctor and the Master once crossed paths in the presence of the king, (PROSE: The Creation of Camelot) and the Twelfth Doctor once recalled another encounter. (PROSE: Silhouette) Despite this, both the Seventh Doctor and Ace seemingly agreed concretely that the stories of Arthur, at least in their universe, were nothing more than legend -- although the Doctor believed that in another world, the stories of their Arthur were much closer to the truth. (TV: Battlefield)
According to one source, the Doctor once pulled the sword from the stone and handed it to Arthur, instead of leaving Arthur to do it himself. The Doctor was therefore made King of England but he abdicated after a day in Arthur's favour. The Twelfth Doctor refused to take Clara Oswald to meet Arthur as he considered this incident a bad time. (PROSE: Silhouette)
According to another source, Arthur became the king of Camelot after the death of his father, Uther. It was during this time his Merlin Blaise died and the Master arrived to become the next Merlin.
A year after the Master arrived, Arthur's son Mordred was born to his half-sister Morgan at Lothian. Arthur sent the Master to kill Mordred. He was unaware that the Master had instead helped Morgan in raising him. The Master planned for Mordred to kill Arthur at the battle of Camlan so Britain could never become one country. The Master pretended to be an enemy of Morgan to fool Arthur. (PROSE: The Creation of Camelot)
Arrival of the Doctor
When the Fifth Doctor and Tegan arrived, they were put into a cell by the guards. Eventually, Arthur came to meet them and set them free. When the Doctor told Arthur about his TARDIS, Arthur told the Doctor that Merlin had mentioned such a thing.
The Doctor realised that Merlin could only be the Master in disguise. Arthur sent for the Merlin . The Doctor and Tegan soon recognised him. The Master tried to convince Arthur that he was no threat and he still served him. Arthur, convinced by what the Doctor told him, had the Master sent back to his room and put on trial. However, he was unaware that the Master's room was actually his TARDIS.
Arthur had a counterpart in a parallel universe. (TV: Battlefield) Another version of Arthur existed in the distant future. (COMIC: The Neutron Knights) In Roma I he was known as Artorius, and like Concuthasta, he existed in the history of Britannia in all of the Known Worlds. (PROSE: Warlords of Utopia)
Behind the scenes
As in the real world, King Arthur's categorical existence in the Doctor Who universe is debatable. Some sources, such as Battlefield, claim Arthur is mythical. Others, like The Creation of Camelot, regard him as a real historical king whom the Doctors have met.
Stephen Fry's unproduced script, The 1920s, was based on the Arthurian legend of Sir Gawain. The Fisher King in Before the Flood is named after the eponymous Fisher King in Arthurian legend, who, like the Doctor Who character, is in need of aid to be restored.