Cwm Llwch, the great glacial valley below and to the north west of Pen-y-Fan is rich with folklore regarding the Tylwyth Teg, 'the little people'.
Below is an extract regarding the valley's circular lake taken from 'The Welsh Fairy Book', W Jenkyn Thomas (1907):
'..In very ancient times there was a door in a rock hard by, which opened once in each year — on May Day — and disclosed a passage leading to a small island in the centre of the lake. This island was, however, invisible to those who stood upon the shore. Those who ventured down the secret passage on May Day were most graciously received by the fairies inhabiting the island, whose beauty was only equalled by their courtesy to their guests. They entertained them with delicious fruits and exquisite music. and disclosed to them many events of the future. They laid down one condition only, and that was that none of the produce of the island was to be carried away, because the island was sacred...'
More of the legend here: