Carreg-y-Bwci or Hobgoblin Stone
I've been trying to find some information on this on the 'net and so far have found two references to it...
The Modern Antiquarian
"The Goblin Stone of Cynwyl Gaio occupied a spot which few people cared to pass at night. In the 17thC a young man who had gone far in search of work came in the twilight to a large stone surrounded with grass. After making a good but simple supper, the traveller placed his bundle containing clothes on the grass in shelter of the stone. For a time he slept soundly, but about midnight he was awakened by somebody pinching his arms and ears and pricking his nose. He got up, and, looking around in the starlight, saw a goblin sitting on the stone, with many others around him. The man tried to run away, but the master goblin would not permit him, and at his command his minions interlaced their grotesque arms around him and prevented him moving. They tweaked his ears and nose, pinched him, gave him pokes in his ribs, and tormented him all through the night in every conceivable manner. He sat down to rest and wait for the dawn, and in the meantime the goblins screamed and laughed and shrieked in his ears until he was nearly mad. When the first streak of morning light appeared, the goblins vanished. The stranger got up in the dawn, and when he went onward he met some workmen, to whom he related his adventure. They said he had slept under the Goblin Stone."
"This is on an alignment which skirts the two sanctuaries and is coincident with a two-mile stretch of the Sam Helen to the north. The line skirts an ancient site to the north of that, as well as going through a few minor points. It is a mound, visible from some distance, and on top is a very large slab of conglomerate atone. There is a hollow in one side which seems to be intended for some kind of offerings.”