Avalon
There have been many theories on the purpose of Avalon. In the book of Taliesin, a poem The Spoils of Annwn says that Arthur descended from Annwn, the Celtic otherworld, with his knights in order to steal a cauldron of plenty. Avalon is said to be the gathering place for departed spirits preparing for Annwn and is guarded by Gwyn ap Nudd, king of the Faeries. The cauldron Arthur planned to steal provided unending nourishment and is known as the original grail he and his knights quested for. Marion Zimmer Bradley’s, The Mist of Avalon, describes Avalon as belonging the last high priestess, Morgaine le Fey. Avalon is said to be the most sacred spiritual place in all of Britain and the keeping place of the Holy Grail. It is the place between the world we live in and the otherworld of Annwn. Avalon is considered to be an omphalos, or a center where one can cross between worlds. Also referred to as the “navel of the world”.
http://earlymedievalbritain.blogspot.com/2009/03/avalon-gateway-to-annwn.html?m=1