This is highly speculative I'm afraid. I know I've got the right place, as I can see the stone marked on maps even as recent as the 1960s. It sounds like an unworked stone, from the extract below. But is it something more interesting?
Maen Y Gweddiau - The Stone of Prayer.
On the Ordnance map, about three or four miles north-east of Coelbren Chapel, among the mountains, Maen y Gweddiau is marked. It is on an open hill, called the Thousand Acres, which is, I believe, private property, and is nothing more than a single flat stone, one of the landmarks between the parishes of Ystradgynlais and Ystradfellte, on which the rector of Ystradgynlais, when perambulating the boundaries of the parish, used to kneel and read prayers to those who accompanied him - hence it is called the Stone of Prayer. The custom has always been observed on every occasion of walking the boundaries, which used to take place every seven years. I could not learn anything as to the origin of the custom, but it is undoubtedly very ancient.
From 'Brecknockshire Traditions' in Archaeologia Cambrensis, April 1858.