Fan Casting King Arthur Pendragon in Shining Knight

Perhaps has gifted Excalibur to Justin for the purposes of the mission? Is still a formidable swordsman and is able to hold his own against Mordred and Morgaine. Does also have some of his forces still within Camelot and also his other knights. Honestly I can't think of how to get Merlin to contact Arthur that he needs his help other than Merlin projecting himself (Astral projection?)