Merlin and the Lost King of England

This story is about the magic.
One day an old man forgot the spell which found that who will become new king.
So he made a mission which found the true King.
The true king don't know himself,too.
The mission is to pull out the sourd from the rock.
The true King only will be able to do it.
There are a lot of challenger to try the mission, but nobody could achieve it.
Who will be able to do it?