Annwn Gwyddno Garanhir Gwyn ap Nudd Tylwyth Teg

Gwyn ap Nudd: horror has a face

And [Gwyn] captured Pen son of Nethog, and Nwython, and Cyledr Wyllt his son, and he killed Nwython, and forced Cyledr to eat his father’s heart, and because of that Cyledr went mad.
(The Mabinogion, translated by Sioned Davies, p209)

There is a truth that needs to be understood about Gwyn ap Nudd, and it is not a comforting truth.