Came here (15.4.2010) direct from the summit of Pen y Gadair Fawr. Postie's directions sum it up, there's no easy path down here from the summit. You can't actually see the stone until you get a little way off the summit itself, although the two appear close together on the map, there is a very large difference in altitude (summit cairn at 800m , stone at 560m). If you are keen/foolish/obsessed enough to follow my route and walk here from Talgarth, directions from the summit cairn as follows:
1. Head SW off the summit cairn. The land drops away very quickly. You will see Pen Cerrig-calch and Pen Allt-mawr on the skyline ahead, and a coniferous forest below them. I didn't think this was Twyn Du at first, it looked too far away and too far below, but it is.
2. Head in the direction of the forest, taking whatever safe route you can. At first the descent is fairly gradual over tussocky grass, but as you head down the slopes get steeper and the vegetation a little higher. You also find yourself in parallel with a number of small streams that started from springs above and you may need to cross one or two of them.
3. As you get lower, the NE edge of the forest becomes visible and with it Maen Llywd comes into view! At least then you know you're in the right place. Carry on down to the stone and be grateful you haven't broken your neck or twisted an ankle getting here. Rejoice!
The stone itself is a lovely flat slab, about 7 feet tall and very narrow on its edges. It sits at the corner of the forest surrounded by a mountainous amphitheatre. Behind you is Pen y Gadair Fawr, ahead is Pen Cerrig-calch (to the SW). The distinctive skyline notch to the NW is the narrow ridge between Pen Trumau and Mynydd Llysiau (at least I think it is!). It echoes, at least slightly, the shape of the top of the stone itself. The stone is very high up - possibly higher than any other in the area, or even Wales(!). Even so, it's still dwarfed by the ridges - none of the summits are visible from this elevation.
Not a soul to be seen, just the odd sheep. Postie is right, really worth the effort. The only (minor) blemish is the tatty rusty fence around the stone, which replaces an older drystone wall intended to separate the forest from the mountainside.
Leaving to the west, the land drops further again along the northern edge of the forest (I couldn't face climbing back up onto the ridge). Although the forest is not access land, there was no sign of any forestry in progress and nothing to indicate that access was forbidden - a large area has been cleared and then left - and the easy downhill walk along forestry tracks provided a simple way back towards Lllanbedr and Crickhowell, including some very pleasant stretches along the Gargwy Fawr and then Grwyne Fechan (there's an interesting ruined Hermitage at the southern end of the forest too).
Looking at the map the stone at first appears quite close to the road, no further than my house to my mums just a 10 minute stroll, however, Pen y Gadair Fawr stands between the carpark and the menhir and its 800metres high. The carpark is at 450m so only another 350 to go. A river must be crossed as well, but a handy bridge is nearby although it then forces you into Mynydd ddu forest criss crossed with good drivable paths, I took the direct route up a barely navigable sometime path through the deep and dark forest, in the end i'd had enough of the forest and came out on the north not far from the summit of Pen y Gadair Fawr. After passing the peak on its southern side I headed down the other side of the mountain 350m below I could just make out the stone by the trees, it seemed really far away and it was then I thought of giving up, but it was downhill all the way so I carried on.
Upon reaching the stone it all becomes worth it, 7-8 feet tall, aligned totally east-west, and is sited in a place of extreme beauty. The stone is thinner on one edge making it a very huggable shape and if you look at it too long it can look like a large woman with her hand on one hip, another hug was in order, one thing lead to another and it was suddenly time to go.
This stone is just about the furthest trek Iv'e ever endured for any single site, and I can't really recomend it had the stone been a tiddler or the view been crap I would have sobbed the whole way back, as it was the trek just built it up for me and the arrival at the journeys purpose was almost euphoric. Viva la Brecon Beacons