In the ancient religions of Indo-European cultures, the idea of an otherworld, also called an otherside, is studied by comparing myths from different cultures. The term comes from the Latin phrase "orbis alius," meaning "other world/side," which was used by the Roman writer Lucan to describe the Celtic Otherworld.

Similar ideas, like a place where spirits live and a place where people go after death, are found in many cultures around the world. In these traditions, spirits are believed to move between different worlds or levels of existence, often following paths such as giant trees, tent poles, rivers, ropes, or mountains.

Indo-European reconstruction

Many ancient cultures, especially those from the Indo-European group, believed in a place called the "Otherworld." In many of these cultures, such as Iranian, Greek, Roman, Germanic, Celtic, Slavic, and Baltic traditions, people had to cross a river to enter the Otherworld. Often, an old man was believed to help souls cross the water.

In Greek and Indian myths, the river’s water was thought to wash away sins or memories. In Celtic and Germanic stories, the river’s water was said to give wisdom, meaning that drinking it could inspire people.

Travelers often met a dog in myths, either as a guardian of the Otherworld or as a guide. For example, in Greek stories, Cerberus, a three-headed dog, guarded the entrance to the underworld. In Indian myths, a dog named Sarvarā, one of Yama’s hounds, may have a name linked to an ancient word meaning "spotted." The Otherworld was shown in many ways, such as peaceful meadows, islands, or buildings, making it hard to know how the earliest Proto-Indo-European people imagined it. The ruler of the dead was possibly Yemo, the twin of Manu, the first human.

In Zoroastrianism, a religious tradition, the Chinvat Bridge, also called the Bridge of Judgement, separates the world of the living from the world of the dead. All souls must cross this bridge after death. It is guarded by two dogs with four eyes each.

A similar idea appears in Hindu mythology, where Yama, the ruler of the underworld, watches the gates with two four-eyed dogs.

In Irish myths, the Otherworld is often described as a peaceful paradise rather than a scary place. Many Celtic stories, such as the Voyage of Saint Brendan and the Voyage of Bran, describe the Otherworld as an island far to the west. Some medieval maps of Ireland even showed this island.

The Celtic idea of the Otherworld mixed with Christian beliefs about heaven and hell, likely because of influences from Roman and Scandinavian cultures. For example, the Voyage of Saint Brendan includes a serpent similar to the Scandinavian Midgard Serpent. Red and white colors were common in the Celtic Otherworld and are still used in religious and political symbols today.

In Germanic myths, apples were closely linked to the Otherworld. Scandinavian myths also describe places connected to the Otherworld, like Valhalla, Odin’s hall, or Ydalar, the home of Ullr. Stories in the Edda and the Draumkvaede describe journeys to the Otherworld.

Early Slavic people believed in a mythical place called Vyraj, where birds flew during winter and souls went after death. This place was often seen as paradise, and it was said that spring began in Vyraj. The gates to Vyraj were guarded by Veles, who sometimes took the form of a creature called a raróg, holding the keys to the Otherworld. Vyraj was also connected to a god named Rod, located far beyond the sea at the end of the Milky Way.

Vyraj was imagined as a garden at the top of a cosmic tree. Birds, believed to be human souls, lived in its branches. As Slavic people converted to Christianity, a new belief spread: one version of Vyraj remained as a heavenly place for birds, while another became an underworld for snakes, linked to the Christian idea of hell.

In Greco-Roman myths, the gods lived on Mount Olympus, while the dead went to the Underworld, Fortunate Isles, or Elysium. In Hades, Cerberus, a three-headed dog, guarded the entrance, preventing the dead from leaving.

Modern depictions

Modern writers have used and changed the idea of the Otherworld in their stories. J. R. R. Tolkien used the Sir Orfeo text, which describes a journey to the Otherworld, as a model for the Mirkwood Elves in The Hobbit. C. S. Lewis used common themes from the Celtic Otherworld in The Chronicles of Narnia, which shows a journey from our world to another. Stephen R. Lawhead’s Song of Albion trilogy tells the story of an Oxford student who moves into the Otherworld.