Where did the “Sword in the Stone” story originate from?
Unknown. Some Arthurian legends don’t seem to know anything about this story. In Norse legend Beorn magically forced three weapons into the rock of a cave wall, and when each of his three sons came of age, each tried to gain a weapon. The youngest, Boðvar-Bjarki, gained the sword. In the Volsunga Saga the god Óðin pushed a sword into an oak tree, which sword could only be removed by the greatest warrior, in this case Sigmund. In Greek mythology after Aegeus of Athens lay with princess Aithra of Troizen, he placed his sword and sandels under an enormous rock, and ordered that if Aithra bore him a son, he should not be told his parentage until he was able to retrieve the sword and sandals. The son, Theseus, did so at age eighteen. Dr. Linda Malcor traces this motif back to Scythian/Sarmatian culture, but her theories are not generally accepted except among her own followers.
