I feel that this cairn ought to be the mound referred to in this story, taken from ‘The Welsh Fairy Book’ by W. Jenkyn Thomas (1908) – online at the sacred-texts archive
sacred-texts.com/neu/celt/wfb/index.htm
There is no end of treasure hidden in the mountains of Wales, but if you are not the person for whom it is intended, you will probably not find it. Even if you do find it, you will not be able to secure it, unless it is destined for you.
There is a store of gold in a hillock near Arenig Lake, and Silvanus Lewis one day took his pickaxe and shovel to find it. No sooner had he commenced to dig in earnest than he heard a terrible, unearthly noise under his feet. The hillock began to rock like a cradle, and the sun clouded over until it became pitch dark. Lightning flashes began to shoot their forked streaks around him and pealing thunders to roar over his head. He dropped his pickaxe and hurried helter-skelter homewards to Cnythog. Before he reached there everything was beautifully calm and serene. But he was so frightened that he never returned to fetch his tools. Many another man has been prevented in the same way from continuing his search.
This may actually refer to Arenig Fawr since there are several hamlets featuring derivatives of the name 'Cynythog' a little way to the east of the larger mountain?