I visited this site back in 2008.
Quite a tricky one to find this one. Take the road south from Pontlottyn and drive through Fochriw. Just as approach the cemetary there is a sharp turning to the right. Park near this junction. Make your way up the steep bank and head for the furthest point north. The cairn is not visible until you are pretty much on top of it. From what I remember there is a post stuck right next to the cairn. The cairn itself is of a reasonable size with stones protuding out of the grass.
Not a lot to see for the effort it takes to find it i'm afraid to say.
Gwladys was one of (Saint) Brychan's many sons and daughters. She was very pretty and attracted the attention of the ruler of the next-door kingdom, Gwynllyw. Gwynllyw asked Brychan if he could marry her, but Brychan wasn't having any of it. Rather impolitely Gwynllyw decided he was going to marry her anyway, so took three hundred of his men over to Brychan's place and snatched her. They rode off in a hurry with Brychan in hot pursuit.
They finally got to Fochriw* which was the border between the two kingdoms. Who should be sitting there playing dice but King Arthur and two of his knights, Cai and Bedwyr. Arthur 'was immediately seized with love towards the lady' and was about to rescue her, but it was pointed out to him that Gwynllyw was now on his own territory, and was persuaded against it (never mind that the poor woman had been kidnapped). In fact Arthur and his knights joined in with rushing against Brychan's men, who ran off. Gwynllyw then took his 'prize' to his palace at Allt Wynllyw (now in Newport).
"Four lamps were seen shining every night with great brightness in the four corners of the house where she remained, until she brought forth her first born son". This was Cadoc, who was later a saint. Gwladys got to be a saint too. It ran in the family.
*also called Vochriw and Boch Riu Carn hill, in Baring-Gould's 'Lives of the British Saints' (Vol 3, 1911).