Annwn (Annwfn) is the underworld of Celtic/Welsh myth, ruled by Arawn (equated with Gwynn ap Nudd). He issued forth on a pale horse to hunt with a pack of red eared hounds. Pwyll, lord of Dyfed ruled Annwn for a year, disposing of Hafgan, Arawn’s sworn enemy during that time.
Annwn is equated in our world with Glastonbury Tor. From this kingdom, Gwydion took pigs, and Arthur stole a magic cauldron from this kingdom. The story of Arthur and the cauldron was created by Taliesin and developed later into the quest for the Holy Grail. According to the this story, Annwn was a land over the sea and so is more closely equated with Ireland.