From Wikipedia:

"Just north of the ridge summit of Cefn Bryn, there is a neolithic burial ground, known as "Arthur's Stone" (Welsh: Maen Ceti).
Its name comes from a legend that the ancient British King Arthur threw a large stone from Llanelli which landed on this spot.

It is believed that during the Bronze Age Cefn Bryn was used extensively for ceremonies and rituals. Beneath the bracken on the hill over sixty cairns have been discovered.
Most of these are likely to be no more than collections of stones cleared by local farmers; but just north-west of Arthur's Stone three prominent cairns have been excavated
that prove their ancient role - the largest being Great Cairn, a circular heap of stones above a central grave".