Annwn or Annwfn, ( "under-world" or "un-world", sometimes inaccurately written Annwyn, Annwyfn or Annwfyn) was the Otherworld, the [[Underworld|land of souls that had departed this world in Celtic mythology.
Entries in the University of Wales' reconstructed Proto-Celtic lexicon  suggest that the name is likely derived from the Proto-Celtic An-dubnion, a phrase with the Proto-Celtic semantic connotations of "extremely deep.", an Abyss. The Modern Welsh spelling 'Annwn' is pronounced an'oon.
Annwn was said to lie so far to the west that not even Manawydan ap Llyr had found it, for you could only reach Annwn by dying yourself. It was also said, though, that Annwn could be entered by those still living if they could find its door.
The door was said to be at the mouth of the Severn near Lundy Island or on Glastonbury Tor. (The temple of Nudd archaeologically discovered near Lydney, and Brythonic stories such as the tale of Seithenyn, suggest that the Severn Bore held symbolic importance in Druid esoteric spiritual teachings. Glastonbury appears widely as a sacred isle of the dead and as the place where saints and kings are buried.)
Pwyll, Prince of Dyfed
In the First Branch of the Mabinogi, entitled Pwyll, Prince of Dyfed, the eponymous prince offends Arawn, ruler of Annwn, by baiting his hunting hounds on a stag that Arawn's dogs had brought down. As a recompense he exchanges places with Arawn for a year, defeating Arawn's enemy Hafgan. Meanwhile, Arawn rules Dyfed. During this year, Pwyll does not sleep with Arawn's wife, earning himself gratitude from Arawn. On his return, Pwyll becomes known by the title Penn Annwn, "King of Annwn."
Culhwch and Olwen
In Culhwch and Olwen, an Arthurian romance associated with the Mabinogion, it is said God gave Gwyn ap Nudd control over the demons lest "this world be destroyed." He led the Wild Hunt. A Christian story tells of the Welsh Saint Collen entering Gwynn's palace to banish him with holy water.
Book of Taliesin
In the Book of Taliesin, an esoteric poem called Preiddeu Annwfn (conventionally translated The Spoils of Annwn) on its face tells a tale of Arthur and his knights traveling through Annwn, searching for a magical cauldron possessed by nine women. Only seven come back from the journey. It may be a precursor of later Holy Grail stories involving King Arthur and his knights. The nine maidens related to actual groups of nine priestesses in ancient Celtic society. Geoffrey of Monmouth told stories of Morgan le Fay and eight other priestesses in his poem, Vita Merlini, who lived on the Isle of Apples or Avalon. Avalon, as an otherworld island, is often identified with Annwn.
Scholars date the spelling of the poem’s text to the 10th century.
As for centuries Welsh poetry was only orally transmitted by bards, it is possible that its original form dates back to the 6th century, when Taliesin is supposed to have lived.