A full account of the ‘Lady of the Lake’ legend is given in W Jenkyn Thomas’ ‘The Welsh Fairy Book’ (1907)... “High up in a hollow of the Black Mountains of South Wales is a lonely sheet of water called Llyn y Fan Fach.....”
However there is an overview for the curious – maybe contemplating a visit? – here:
sacred-texts.com/neu/celt/wfb/wfb03.htm