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The concept of an "otherworld" in historical Indo-European religion is reconstructed in comparative mythology. The term is a calque of orbis alius or "Celtic Otherworld", so named by Lucan in his description of the druidic doctrine of metempsychosis.
Comparable religious, mythological or metaphysical concepts, such as a realm of supernatural beings and a realm of the dead, are of course found in cultures throughout the world. Spirits were thought to travel between worlds, or layers of existence, usually along an axis such as a giant tree, a tent pole, a river, a rope or mountains.
Many Indo-European mythologies show evidence for a belief in an "Otherworld" and in many cases such as in Greek, Germanic, Celtic, Slavic and Indic mythologies a river had to be crossed to allow entrance to the Otherworld and it is usually an old man that would transport the soul across the waters. In Greek and Indic mythology the waters of this river were thought to wash away sins or memories whereas Celtic and Germanic myths feature wisdom-imparting waters, suggesting that while the memories of the deceased are washed away a drinker of the waters would gain inspiration. The wayfarer will commonly encounter a dog either in the capacity of a guardian of the Otherworld or as the wanderer's guide. Examples of this are the Greek Cerberus, the three-headed hound of Hades, and the Indic सर्वरा "sarvarā, one of the hounds of Yama, whose names may derive from an Indo-European *ḱerberos meaning "spotted". In Indo-European mythologies the Otherworld is depicted in many ways, including peaceful meadows, islands and buildings making it hard to determine how the original Proto-Indo-European Otherworld was viewed. However the ruler of the dead was possibly Yemo, the divine twin of Manu the first man.
As was the case in the Celtic mythologies, in Germanic myths apples were particularly associated with the Otherworld. In the Scandinavian tradition mythological localities are featured, as in Irish mythology; however, unlike Irish mythology, an attempt was made to map the localities of the Otherworld rather than list locales associated with it. In the Edda many locations are named including the dwellings of the gods such as Odin's hall of Valhalla or Ullr's dwelling of Ydalar ("Yewdale"). The Gylfaginning and the later Norwegian poem the Draumkvaede feature travels into the Otherworld.
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