King Arthur

King Arthur was the king of Camelot.

Coronation
Arthur became the next king to rule Camelot after the death of his father Uther. It was also during this time his Merlin Blaise had died and the Master arrived and became the next Merlin.

A year after the Master arrived, Arthur's son Mordred was born by his half-sister Morgan at Lothian. Arthur sent the Master to go out and kill Mordred however unaware that the Master had instead helped Morgan in raising him so for him to kill Arthur at the battle of Camlan so Britain could never become one country. The Master pretended to be enemies with Morgan to fool Arthur. (DWA: 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 which the Doctor and Tegan soon recognised him. The Master tried to convince Arthur that he was no threat and that he still served him. Arthur however, convinced by what the Doctor told him, had the Master to be sent back to his room and to be put on trial. However, he was unaware that the Master's room was actually his TARDIS.

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