In Welsh mythology, Gwyn ap Nudd was the ruler of Annwn (the Underworld) and escorted the souls of the dead there, leading a pack of supernatural hounds, called the Cŵn Annwn (Hounds of Annwn) (see also Wild Hunt).
Bébinn is also described as being an underworld goddess in both Irish and Welsh mythology, inhabiting either the Irish underworld Mag Mell or the Welsh Annwn, although it is unknown which is the original source.
Exhausted from his journey, Crom falls asleep, and dreams a horrific encounter with Arawn, the king of the netherworld realm of Annwn.
Collaboration on two further operas, The Children of Don (first performed at the London Opera House, conducted by Thomas Beecham, on 12 June 1912) and Bronwen, brought about the completion of Holbrooke's most ambitious project, a trilogy under the collective title The Cauldron of Annwn setting Scott-Ellis' versions of tales from the Welsh Mabinogion.
According to the Book of Taliesin, he fought alongside Gwydion at the Battle of the Trees, in which he assisted his uncle in enchanting the trees to rise up in battle against Arawn, king of Annwn.
Pwyll, the prince of Dyved, offends Arawn, ruler of Annwn (the underworld), by baiting his hunting hounds on a stag that Arawn's dogs had brought down.
The Cauldron of Khath Meigol and its powers of resurrection hark back to Welsh mythology's tale of the Cauldron of Annwn, and a number of the deities seem to have Celtic or Welsh roots: Paul is known as Pwyll after his sacrifice, while Macha and Nemain come straight from Irish mythology.