Mabon ap Modron's Epitaph
The grave in the upland of Nanllau;
His story no one knows.
Mabon the son of Modron the sincere.
--Myv. Arch. I. p. 78. This stanza, derived from the semi-reliable (but no less important) Myvyrian Archaiology. It doesn't appear in the BBC's "The Stanzas of the Graves", which leads one to wonder if it is one of Iolo Morgannwg's forgeries; however, it could also easily be from an overlooked manuscript, a currently-missing manuscript, or some such thing.
SOURCE: Guest, Charlotte. The Mabinogion. nd. London: J.M. Dent.