An abstract spiritual world beyond earthly reality.

The Otherworld (Latin: "orbis alia") in Celtic mythology is postulated (but not known) to be the realm of the dead, the home of the deities, or the stronghold of other spirits and beings such as the Sídhe. Tales and folklore describe it as existing over the western sea, or at other times underground (such as in the Sídhe mounds) or right alongside the world of the living, but invisible to most humans.

The Irish believed in an Otherworld, which they described sometimes as underground, such as in the Sídhe mounds, and sometimes located on islands in the Western Sea. The Otherworld was variously called Tír na mBeo ("the Land of the Living"), Mag Mell ("Delightful Plain"), and Tír na nÓg ("Land of the Young"), among other names. It was believed to be a country where there was no sickness, old age, or death, where happiness lasted forever, and a hundred years was as one day. It was probably similar to the Elysium of the Greek mythology and both may have a shared origin in ancient Proto-Indo-European religion. In Irish Immrama ("voyage") tales, a beautiful young woman often approaches the hero and sings to him of this happy land. Sometimes she offers him an apple, or the promise of her love in exchange for his assistance in battle. He follows her, and they journey over the sea together and are seen no more. Their journey may take place in a boat of glass, in a chariot or on horseback (usually upon a white horse, as in the case of the goddess Niamh of the Golden Hair). Sometimes the hero returns after what he believes is a short time, only to find that all his companions are dead and he has actually been away for hundreds of years. Sometimes the hero sets out on a quest, and a magic mist descends upon him. He may find himself before an unusual palace and enter to find a warrior or a beautiful woman who makes him welcome. The woman may be the goddess Fand, the warrior may be Manannán mac Lir or Lugh, and after strange adventures the hero may return successfully. However, even in cases where the mortal manages to return to his own time and place, he is forever changed by his contact with the Otherworld.

This conception of the Otherworld is also preserved in the Welsh story of Branwen, daughter of Llyr, which ends with the survivors of the great battle feasting in the presence of the severed head of Bran the Blessed, having forgotten all their suffering and sorrow, and having become unaware of the passage of time. In Irish lore, Donn, a god of the dead, reigned over Tech Duinn ('The House of Donn'), which was seen as existing on or under Bull Island, located off the Beare Peninsula in the southwest of Ireland. It was believed that the newly-dead journeyed to Tech Duinn, either to remain there forever, or perhaps as a starting-point on their journey to the Blessed Isles across the Western Sea. A Welsh corollary to Tech Duinn is Annwfn, ruled by the Otherworld kings Arawn and Gwyn ap Nudd.

Celtic folklore and mythology are full of tales that tell of humans wandering into the Otherworld, and of supernatural beings crossing over into the human realm. This is considered most likely to happen at particular, liminal places, or on special days of the year. For instance, on the Gaelic festival of Samhain (November 1, seen by many as the Celtic New Year), it is believed that the boundaries between the worlds become even more permeable than usual, and mortals might cross over to the spirit world - usually accidentally - and the inhabitants of the spirit world might cross over into the mortal realm. The spring festival of Beltane (May 1) is also seen as a time when the Otherworld is particularly close at hand.

Traditionally, much folklore and folk practice is concerned with preventing the intrusion of spirits into this world, or the loss of humans to the Otherworlds, and many charms and taboos exist for protection from these incidents. Some of this is seen in fairy lore, where humans fear the fairies might steal human babies and leave Changelings in their places. However other traditions think more kindly of the fairies and other spirits, and encourage the leaving of offerings for them, such as milk and baked goods, in order to form a treaty or friendship with them.

The Otherworld has been described in Irish poetry and tales as being a land of paradise, happiness, and summer. It is often described as a series of islands where the various deities and ancestors live. Many mythological heroes, such as Cúchulainn and Bran in The Voyage of Bran, journeyed to Otherworld realms.

In the First Branch of the Welsh tales known as the The Mabinogion, entitled Pwyll, Prince of Dyfed, the eponymous prince offends Arawn, ruler of Annwn (the Welsh Other World), by baiting his hunting hounds on a stag that Arawn's dogs had brought down. In recompense he exchanges places with Arawn for a year and defeats Arawn's enemy Hafgan. Meanwhile, Arawn rules Dyfed. During this year, Pwyll does not sleep with Arawn's wife, earning himself gratitude from Arawn. On his return, Pwyll becomes known by the title Pen Annwn, "Head (or Ruler) of Annwn."

The Irish tradition tells that the fairies are descendants of the Tuatha Dé Danann, an ancient folk that were driven to the underworld by a wave of invaders, the Gaels, who came from Spain led by chieftain Míl Espáine.

The Tuatha had no other choice than to take refuge under the Sídhe, a Celtic word which denotes the hills where the long barrows lay and which is used also to name a special kind of fairies in Ireland and in the Scottish Highlands, the daoine Sídhe. All through Ireland legends can be heard about Knocks (from the Irish cnoc), hollow hills which are inhabited by extended fairy communities ruled by a King or a Queen. The best known Sídhe sites in Ireland are: Knockma, where the throne of Finvarra (King of the fairies of Connaught) is located, Knockany, ruled by Ainé, Queen of Munster, and Newgrange in county Meath, a megalithic building which is associated with the Angus Óg myth.

The Sídhe can be found by humans in certain times in the year, especially at Midsummer, when they are seen often dancing under the moonlight.

In Brittany and in Asturias similar myths are kept. In the Asturian mythology there are many stories who describe human encounters with xanas (fairies), which are dancing around one of them, the Xana Mega, the Queen of Fairies. The castro of Altamira is said to hide an enormous underground realm which is ruled by a royal couple, and whose entrance is found someplace on the hill.

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