As Earthstepper says, Dinas Bran has been suggested as the resting place of the holy grail – but this hill of the fairies is also connected with another treasure. Gold – maybe a golden harp, specifically – will be found by a boy who is led to it by his white dog with a silver eye. Dogs like that can see the wind, you know. Susan Cooper uses the theme in her book ‘The Grey King’.
It’s also possible that the St Collen story now associated with Glastonbury Tor ( themodernantiquarian.com/post/26342 ) was originally intended to be set somewhere round here (after all, St Collen spent plenty of time here and the town below, Llangollen, is named after him).