Preiddeu Annw(fy)n (Spoils of Annwn), from Llyfr Taliessin (Book of Taliesin). early Fourteenth Century AD, originally Ninth Century AD (c AD 900):

‘Preiddeu Annwn: The Spoils of Annwn by Sarah Higley (translator)’ at The Camelot Project, 2007.
https://d.lib.rochester.edu/camelot/text/preiddeu-annwn