When Merlin had done most of what he wanted there, he went away and left the island like this. And he did still more. For he made there a bridge of iron, which was not half a foot wide^ and stretched from one bank to the other. He said that by this would one be able to know the bold knights, for only the boldest would ever have the courage to cross over by this bridge. At the end of the bridge toward the castle, where the crossing began, he had a block of marble placed; within this block he fixed a sword by magic and put the scabbard of the sword in such a way that you would have thought that nothing held it and that one could take it easily, but one could not.

Then he made marvelous letters in the pommel of the sword which said, HE WHO FIRST TRIES TO LIFT THIS SWORD FROM HERE WILL BE WOUNDED BY IT FIRST. And it happened just as he said, shortly after Galahad the good knight came to King Arthur’s court, for Gawain tried it first, at his uncle’s urging, and was wounded by it first, as the history will tell openly.

Then he wrote in the block itself other letters that said, NEVER WILL THIS SWORD BE LIFTED FROM HERE EXCEPT BY THE HAND OF THE BEST KNIGHT IN THE WORLD, NOR SHOULD ANYONE HURRY TO LIFT IT, UNLESS HE FEELS HIMSELF THE BEST KNIGHT OF ALL, FOR IT WILL BE BAD FORTUNE FOR HIM. Then he pushed the stone and put it in the water and performed so many spells that it then went floating on the water a long time and was seen in many countries; it went so from place to place until it came to Camelot after a long time and arrived at the city gate the very day Galahad first came to court.

Post-Vulgate Merlin