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 found in cultures throughout the world. Spirits are thought to travel between worlds, or layers of existence in such traditions, usually along an axis such as a giant tree, a tent pole, a river, a rope or mountains.
Otherworld 1985 s01e01 rules of attraction
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.
Many Celtic Immrams or "voyage stories" and other medieval texts provide evidence of a Celtic belief in an otherworld. One example which helps the reader understand the Celtic concept of the otherworld is The Voyage of Saint Brendan. Another Classic example of a Celtic "otherworld" is the Voyage of Bran. Because Celtic life largely was based upon nourishment from the sea and around the wet and foggy weather of Northern Europe the otherworld is often portrayed as an island to the west in Celtic oral tradition and even shown on some maps of Ireland during the medieval era. The otherworld in the idea of Celtic people became hard to distinguish and sometimes overlapped with the Christian idea of hell or heaven as this was often an analogy made to the Celtic idea of an otherworld or Scandinavian idea of a world tree. This is likely because of Roman and Scandinavian influences on Celtic cultures. An example of Scandinavian influence is apparent in the Voyage of Saint Brendan from the likeness of Lasconius the serpent to the Scandinavian Midgard Serpent. Red and white are the colors of animals in the Celtic Otherworld, and these colors still animate transcendent religious and political symbols today.
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.
In Greco-Roman mythology the Gods were said to dwell on Mount Olympus whereas the dead usually went to the Underworld or Fortunate Isles after death.