Fate/Pendragon

In a parallel world, the descendant of the great King Arthur himself rules over modern Camelot. However, unlike the previous generations, he holds power unlike any other. Having always felt connected to the Throne Of Heroes, he has the power to call forth a number of Noble Phantasms. Through a twist of unexpected events, he finds himself stranded in a world that is stuck within medieval times. What kind of effect will he have in this fantasy world?