King Caerleon was a supporting antagonist in the fourth season of BBC's Marlin.
Sometime after King Uther Pendragons demise, King Caerleon and his men were invading the bordering villages around Camelot. After being captured, Sir Agravaine convinced Arthur Pendragon to force The King into signing up his treaty. However the king's refused and Arthur was convinced to execute him. This incurred the wrath of Caerleon's wife, Queen Annis, who vowed to destroy Arthur Pendragon for his "crime."
Arthur realized his mistake and eventually made amends with Queen Annis after proving himself to be a selfless leader when he spared her champion during the duel.