Arthurian Otherworld Also in British and Welsh, particularly those concerning King Arthur, such as the Isle of Avalon, the domain of the Lady of the Lake and the Grail Castle, can all be considered to be Celtic Otherworld.
Avalon was like the "Isles of the Blessed", has been called "Isle of Apples". Avalon was derived from the name, Ynys Afallon, in the Welsh myth. When King Arthur was mortally wounded, his half-sister, Morgan le Fay, writers say that she brought the king to Avalon, to be healed. Geoffrey of Monmouth, mentioned Morgan le Fay as one of nine sisters who were also great sorceresses and they lived in Avalon, but Geoffrey does not connect her being Arthur's sister. See Battle of Camlann (Early Traditions) and Twilight of the Kingdom, for a few different variations about the episode of death of Arthur and Avalon. Layamon wrote that the Avalon was ruled by elf-queen named Argante, who could be another name for Morgan, since later authors had often called her the ruler of Avalon. In Erec and Enide (c. 1165), Chretien de Troyes wrote that one of the wedding guests was Guingamar. Guingamar was the lord of Avalon and a friend of Morgan le Fay. In the early thirteenth century, some believed that Avalon was situated in Glastonbury. The monks or priests in Glastonbury wrote their own version of the Grail legend, called Le Haut Livre du Graal or Perlesvaus. They even claimed that Arthur and Guinevere were buried in Glastonbury. Gerald of Wales, who wrote his Tour of Wales in the late 12th century, had visited the site and also believed that Glastonbury was the burial sites of Arthur. Glastonbury was situated on the island in the middle of a marshland. Most contemporaries and modern scholars were sceptical about these claims of the Avalon/Glastonbury connection. Glastonbury Tor was also sometimes called the "Isle of Glass" or "Tower of Glass". The name is similar to the Welsh Caer Wydyr or "Fortress of Glass".
Another famous Arthurian Otherworld was the domain of the Lady of the Lake. Some say her home was an underwater palace, while others say that the lake was only a powerful illusion to hide her home from intruders. The heroes and cousins, Lancelot and Bors were brought up in her home. Lancelot gained the name Lancelot of the Lake, because of his association with the Lady of the Lake. (See Lady of the Lake in Lancelot du Lac.) It was the Lady of the Lake who gave Arthur the Otherworldly sword, Excalibur. (see Legend of Excalibur)
The magical forest and spring of the Lady of the Fountain could also be considered to be an Otherworld. The water in the spring looked like it was boiling as it bubbles, yet the water was actually cold. Pouring the water from a golden bowl on to huge marble stone would cause an immediate storm to lash the forest and the Lady's castle. Some Otherworld may look like paradise, but was actually some elaborate prison. As in the case of the Valley of No Return (Le Val Sans Retour). Here, no knight could leave the land, if he had ever been unfaithful to his true love. The entire valley was like paradise, where valley was green and there was always plenty of water, even though it never rain. There was always feasting and dancing. Lancelot broke the spell, because he had always been true to love - his love for Guinevere.