King Arthur is a legendary King and wielder of the sword Excalibur. He was the head of the Knights of the Round Table. He and his knights fought various enemies. Before King Arthur died, he had one of his knights give Excalibur back to the Lady of the Lake until the day he returns and he can reclaim it.
In an attempt to defeat the ThunderCats, Mumm-Ra disguises himself as King Arthur in order to claim Excalibur.