Saint Illtyd was King Arthur’s cousin and helped out with his funeral arrangements, according to the medieval writings of Nennius.
You can read about his rather eventful life at Mary Jones’ page:
geocities.com/branwaedd/illtud.html
One of his deeds involved turning two robbers into standing stones. Perhaps he did use this tomb as a hermitage – all the crosses carved on it suggest a lot of people believed he did.
He himself is supposed to be buried on Mynydd Illtud in the Brecon Beacons (apparently close to the visitor’s centre :)