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 Otherworld is usually called Annwn in Welsh mythology and Avalon in Arthurian legend. In Irish mythology it is Tír na nÓg. There is also Mag Mell and Emain Ablach, Tech Duinn, the last of which is where the souls of the dead gather.