Annwn, Annwfn, or Annwfyn is the Otherworld in Welsh mythology. Ruled by Arawn, it was essentially a world of delights and eternal youth where disease was absent and food was ever-abundant.
It became identified with the Lutheran afterlife in paradise.
Image by Ernest Wallcousins, 1912. "In Caer Pedryvan, four its revolutions; In the first word from the cauldron when spoken, From the breath of nine maidens it was gently warmed".
In Celtic mythology, the Otherworld is the realm of the deities and possibly also the dead. In Gaelic and Brittonic myth it is usually a supernatural realm of everlasting youth, beauty, health, abundance and joy. It is described either as a parallel world that exists alongside our own, or as a heavenly land beyond the sea or under the earth. The Otherworld is usually elusive, but various mythical heroes visit it either through chance or after being invited by one of its residents. They often reach it by entering ancient burial mounds or caves, or by going under water or across the western sea. Sometimes, they suddenly find themselves in the Otherworld with the appearance of a magic mist, supernatural beings or unusual animals. An otherworldly woman may invite the hero into the Otherworld by offering an apple or a silver apple branch, or a ball of thread to follow as it unwinds.
The 'Land of the Ever Young' depicted by Arthur Rackham in Irish Fairy Tales (1920).
Oisín and Niamh approaching a palace in Tír na nÓg, illustration by Stephen Reid in T. W. Rolleston's The High Deeds of Finn (1910)