Celtic Otherworld
In Celtic mythology, the Otherworld is the realm of the deities and possibly also the dead. In Gaelic and Brittonic myth it is usually described as a supernatural realm of everlasting youth, beauty, health, abundance and joy. The Otherworld is similar to a parallel universe, 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, the Otherworld is said to exist alongside our own located beyond  the edge of the earth and intrudes into our world; signalled by phenomena such as magic mist, sudden changes in weather, or the appearance of divine 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 has several names, including "Tír na nÓg, Mag Mell" and "Emain Ablach". In Irish myth there is also "Tech Duinn", where the souls  of the dead gather. Irish mythology. In Irish mythology, the Otherworld has various names. Names of the Otherworld, or places within it, include "Tír nAill" ("the other land"), "Tír Tairngire" ("land of promise/promised land"), "Tír na nÓg" ("land of the young/land of youth"), "Tír fo Thuinn" ("land under the wave"), "Tír na mBeo" ("land of the living"), "Mag Mell" ("plain of delight"), "Mag Findargat" ("the white-silver plain"), "Mag Argatnél" ("the silver-cloud plain"), "Mag Ildathach" ("the multicoloured plain"), "Mag Cíuin" ("the gentle plain"), and "Emain Ablach" (possibly "isle of apples"). It is described as a supernatural realm  where there is everlasting youth, beauty, health, abundance and joy, and where time moves differently. It is the dwelling place of the gods (the Tuatha Dé Danann) as well as certain heroes and ancestors. It was probably similar to the Elysium of Greek mythology and both may have a shared origin in ancient Proto-Indo-European religion. The Otherworld is elusive, but various mythical heroes—such as Cúchulainn, Fionn and Bran—visit it either through chance or after being invited by one of its residents. In Irish myth and later folklore, the festivals of Samhain and Beltane are liminal times , when contact with the Otherworld was more likely. In the tales, the Otherworld is often reached by entering ancient burial mounds, such as those at Brú na Bóinne and Cnoc Meadha. These were known as "sídhe" ("Otherworld dwellings") and were the dwellings of the gods, later called the "aos sí" or "daoine sí" ("Otherworld folk"). Irish mythology says the gods retreated into the "sídhe" when the Gaels (Milesians) took Ireland from them. In some tales, the Otherworld is reached by going under the waters of pools, lakes, or the sea, or else by crossing the western sea. In Irish "Immrama  (voyage") tales, a beautiful young Otherworld 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 help in battle. He follows her, and they journey over the sea together and are seen no more. Their journey may be in a boat of glass, in a chariot, or on horseback (usually on 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 when the mortal manages to return to his own time and place, he is forever changed by his contact with  the Otherworld. The Otherworld was also seen as a source of authority. In the tale "Baile in Scáil" ("the phantom's ecstatic vision"), Conn of the Hundred Battles visits an Otherworld hall, where the god Lugh legitimizes his kingship and that of his successors. In Irish myth there is another otherworldly realm called "Tech Duinn" ("House of Donn" or "House of the Dark One"). It was believed that the souls of the dead travelled to "Tech Duinn"; perhaps to remain there forever, or perhaps before reaching their final destination in the Otherworld, or before being reincarnated. Donn is portrayed as  a god of the dead and ancestor of the Gaels. "Tech Duinn" is commonly identified with Bull Rock, an islet off the west coast of Ireland which resembles a portal tomb. In Ireland there was a belief that the souls of the dead departed westwards over the sea with the setting sun. West-ward also being the location of the phantom island, anglicised as, Hy-Brasil. Welsh mythology. In Welsh mythology, the Otherworld is usually called "Annwn" or "Annwfn". The Welsh tale of "Branwen, daughter of Llyr" ends with the survivors of the great battle feasting in the Otherworld, 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. "Annwn" is ruled by the Otherworld kings Arawn and Gwyn ap Nudd. In the First Branch of the Welsh tales known as the Mabinogi, entitled "Pwyll, Prince of Dyfed", the eponymous prince offends Arawn, ruler of Annwn, by baiting his hunting hounds on a stag that Arawn's dogs had brought down. In recompense, Pwyll swaps 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". Continental Celtic mythology. The Gauls divided the universe into three parts: "Albios" ("heaven, white-world, upper-world"), "Bitu" ("world of the living beings"), and "Dubnos" ("hell, lower-world, dark-world"). According to Lucan, the Gaulish druids believed that the soul went to an Otherworld, which he calls by the Latin name "Orbis alius", before being reincarnated. Greco-Roman geographers tell us about Celtic belief in islands consecrated to gods and  heroes. Among them were Anglesey ("Môn"), off the north coast of Wales, which was the sacred isle of the druids of Britain; the Scilly isles, where archaeological remains of proto-historical temples have been found; and some of the Hebrides, which were, in the Gaelic tradition, home to ghosts and demons: on one of them, Skye, the Irish hero Cúchulainn was taught by the warrior woman Scathach. Byzantine scholar Procopius of Caesarea described the Otherworld of the ancient Gauls. He said it was thought that the land of the dead lay west of Great Britain. The Continental Celtic myths told  that once the souls of the dead had left their bodies, they traveled to the northwest coast of Gaul and took a boat toward Britain. When they crossed the Channel, the souls went to the homes of the fishermen, and knocked desperately at their doors. The fishermen then went out of their houses and led the souls to their destination in ghostly ships. There are still remains of those beliefs in the folklore of Brittany, where the name "Bag an Noz" is used to denote those ships who carry the dead to their goal: Anatole Le Braz describes in his  book "La légende de la mort chez les Bretons armoricains" the existence of souls' processions which make their way toward coastal places like Laoual, to start their last travel from there. In Asturian mythology, there are many stories which describe human encounters with "xanas", fairies which are dancing around a chief fairy, the "Xana Mega", or the "Queen of Fairies", known as in Galicia. The "castro" of Altamira is said to hide an enormous underground realm which is ruled by a royal couple, and whose entrance is found some place on the hill.