King Arthur

King Arthur was the king of Camelot.

Coronation
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. (DWAN: The Creation of Camelot)

Arrival of the Doctor
When the 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 the Merlin had mentioned such a thing.

The Doctor realised that the 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.

After the Master escaped, the Doctor suggested Arthur (in order to build the Arthurian legend) create the Knights of the Round Table. (DWAN: The Creation of Camelot)