Some folklore about a cave under Craig Rhiwarth, recorded in ‘Celtic Folklore – Welsh and Manx, by Rhys (1901). Cwm Glanhafan is on the mountain’s eastern side.
Take for instance a cave in the part of Rhiwarth rock nearest to Cwm Llanhafan, in the neighbourhood of Llangynog in Montgomeryshire. Into that, according to Cyndelw in the Brython for [date missing on STA], p. 57, some men penetrated as far as the pound of candles lasted, with which they had provided themselves; but it appears to be tenanted by a hag who is always busily washing clothes in a brass pan.
Online at the Sacred Texts Archive
sacred-texts.com/neu/cfwm/cf202.htm