The legend relates to one of Arthur's tasks: to rid the western Brecon Beacons of the pack of wild boars that were terrorizing the people. Arthur chased the boars from Dyfed eastward towards Powys. On the Black Mountain, he picked up a large stone (the carreg fryn fras) and cast it towards the wild animals, striking dead the leader of the pack on the edge of a valley near Craig-y-Fran Gorge. The big boar's body rolled down the valley and into the river which is now the Afon Twrch. The big stone is still on the mountain.
