The Celtic Otherworld, akin to Hades or Hell, inharited by fairies, demons, maidens, and warriors. It is variously described as an island, a valley, and an underworld. It is said to have various portals of entry from the mortal world.
An early Welsh poem, Preiddeu Annwfn, describes an expedition by Arthur and his warriors to the mysterious otherworld, where they defeat 600 warriors and obtain a magic cauldron kept by nine maidens. The narrator of the story is Taliesin, one of those who took part in the expedition. Those involved sailed overseas in three boatloads to reach their goal. They reached the fort or city of the Otherworld, called by a number of names (Caer Rigor, Caer Siddi, etc), but only seven men returned. The references to the ships, including Arthur’s ship Prydwen, suggests that Annwn is an island.
According to Culhwch and Olwen, the powers of the demons of Annwn were vested in the warrior Gwynn, son of Nudd. The language of the poem is obscure. In the poem, Annwn is called, among other things, a fairy fortress, a glass fortress, and a “fort of carousal”. The expedition of Arthur to Ireland in Culhwch and Olwen may be another version of this story, Ireland being substituted for the Otherworld. It is not impossible that the original story told how Arthur obtained Excalibur from the Otherworld.
Culhwch and Olwen | Late 11th century
Preiddeu Annwfn | Attributed to Taliesin, c. 900