The 2,608ft summit of Cribyn (sometimes also referred to as Cribin) is generally considered in South Wales to bear the remnants of a Bronze Age burial cairn. Indeed, situated as it is between two excavated examples upon nearby Pen-y-Fan and Corn Du to the west, and an excavated cremation burial upon Fan y Big to the east, it would (arguably) have been very unlikely not to have been chosen in this respect - particularly bearing in mind Cribyn's topographical profile and status as one of the Breacon Beacons' 'Big Three'.
The aforementioned Bronze Age cairns gracing Pen-y-Fan and Corn Du were subsequently protected - following excavation - from further erosion through the construction of overlying modern cairns; not so Cribyn, which, as far as I'm aware remains unexcavated and at the mercy of walker's boots. It is now surmounted by a feeble 'walker's cairn' and, according to a recent survey by The Clwyd-Powys Archaeological Trust, considered to be at great risk of being lost for ever:-
CPAT Project - Prehistoric Funerary and Ritual Sites, 2005/06: Breconshire:
"....encouragingly, the majority of sites (86%) are considered to be subject to a low impact threat, with 12% subject to a medium impact threat, and only one site with a high impact threat, which is the summit cairn on Cribyn (PRN 4560)."
It therefore seems somewhat paradoxical to encourage visitors to this fragile site ... but since this monument is currently sitting in the realms of 'who gives a monkey's land', raising public awareness, I think, is the only way forward. If you fancy it - and bear in mind this is a serious mountain walk requiring all standard precautions - The AA have published walk guidance notes at:
Incidently, these state that "The summits of Pen y Fan, Corn Du and Cribyn were once all crowned with Bronze-Age burial cairns, probably dating back to around 1800 bc. It's clear that the mountains held some significance, even way back then".