King Mark the Rightful is the King of Dumonia and the second recruitable Knight for King Arthur's Round Table. King Mark will join Arthurs Round Table after you defeat Sir Glais, King Indres Knight and the volunteers quest will appear once you control Dumonia. King Mark is a Rightful Knight and less of an Ideal choice for a Tyrant player and is also a Christian King which will cause some loyalty issue's unless you get him to convert to the old faith via various Quests that add to the old faith of the Knight.
Seeing as he is a King it is likely that King Mark was present when Arthur pulled the sword from the stone.