"'From Annwfn,' he replied, 'I am Arawn, king of Annwfn.'"
St. Michael's Mount at sunset
Creative Commons Attribution Share AlikeSt. Michael's Mount at sunset - Credit: Bob Jones

Annwfn or Annwn is the Welsh name for the Celtic Otherworld. Following the advent of Christianity it was also used as a word for hell. Annwfn/Annwn may be translated either as ‘non-world’ or ‘inner-world’. However, the exact nature of the concept, and the imagined location of the Otherworld, is not entirely clear, either in the Welsh, or other Celtic traditions. 

An early medieval Welsh poem known as Preiddiau Annwfn ('The Spoils of Annwfn') describes a visit by King Arthur and his men to various Otherworld 'fortresses', but their significance remains obscure. The Celtic Otherworld is sometimes described as being situated on an island, often the Isle of Avalon (Ynys Afallon, in Welsh). Various real places have been suggested as the location of Avalon, ranging from Glastonbury and Bardsey Island to St. Michael’s Mount in Cornwall. In the Irish tradition, the Otherworld has been given the names Tír na n-Óg ('The Land of the Young'); Tir Tairngire ('The Land of Promise') and Tir na t-Samhraidh ('The Land of Summer').