What scheme does Merlin come up with in order to reveal Britain's nee king?

Merlin devises a scheme involving a sWord embedded in a stone, proclaiming that only the rightful king can pull it out. This act serves as a test to identify the true ruler of Britain. When young Arthur, unbeknownst to his royal lineage, manages to draw the sWord from the stone, it reveals him as the legitimate king. This moment marks the beginning of Arthur's reign and the fulfillment of Merlin's prophecy.