King Aruthur was the leader of Britain, the husband of Queen Guinevere, and father of Halloween (then Princess Mordred). He separated Guinevere and Halloween, and was going to burn Guinevere for witchcraft when Lancelot, his best knight, saved her.
King Arthur is now deceased, causing Merlin to request Tasha Godspell to release his contract with Halloween to reassert her as the Queen of England.