The Exodus Myth

Myths of a mass exodus from three of the Five Realms to Alterra exist in every culture which acknowledges the realms in some form. The story goes hand in hand with that of the Withering, an apocalyptic event which affected Jitua, Hjemtir and Ekhanua. So the tales go, refugees from those worlds poured into the region which is now known as the city of Lokmare via portals.
While few outside of the region believe this, Lokmare has attracted interest from the likes of anthropologists and linguists for the fact that a number of cultures very distinct from each other all appeared in that region from nowhere at around the same time as each other, an anomaly that so-called "rational" academics cannot explain.

Summary

Nobody knows exactly what caused the Withering--in fact most people don't really know what it was, except it caused mass destruction in those specific realms--but it swallowed up everything in its path and those who made it to Alterra were the lucky few. Whether anything survives back in those worlds is anyone's guess ...
In each of the affected realms, there were individuals who knew of the existence and location of at least one portal, and helped others to safety. Each portal only leads to one other realm, so many had to do trial and error before they found a realm unaffected by the Withering (Alterra). The refugees were made up of anything from a single individual to entire tribes. While they were coming from three different worlds, they all poured into the same region of Alterra, which is cosmically connected to the other realms. (Though portals elsewhere in Alterra do exist, they're just much rarer.)
Some parts of the land was inhabited by the native Eniwen, but there was enough space for groups from the other realms to settle and create their own communities. Over decades and centuries these communities grew and spread out over the land, eventually swallowing each other up and becoming one large city, to be named Lokmare.
Knowledge of what their ancestors went through and their origins fell into myth and became fairy tales and ghost stories. Even those who are aware of, or even practise, magic, don't necessarily believe in the existence of the Five Realms, much less that their family was once from one of the others.
While many aspects of their refugee ancestors' cultures remain, including some languages, festivals, architectural styles and miscellaneous traditions, years of such a diverse mix of minority cultures living so closely together has had very different effects. Some languages which came into Alterra from other realms are now extinct, three holidays from different religions were adapted into one, and there have been many periods of tension between different people groups within Lokmare throughout the city's history. However, Lokmare's uniqueness has often been its strength, modern Lokmare encourages the embracing of differences and rediscovering of old traditions, and although Omerton is the capital of the Ewenin Nations, in practise much of its diplomatic business is conducted in Lokmare, due to the city's reputation.

Historical Basis

The Withering was very real, as was the mass exodus from Hjemtir, Jitua and Ekhanua. All three realms survived and are still populated today. The portals are also still functional, meaning occasionally, someone from one realm visits another.

Cultural Reception

For some, whether they believe it or not, the story of the exodus is one which brings comfort and camaraderie. It serves as a reminder of what they have to be grateful for, and a sense that they share something in common with others who have the same story in their own ancestry.
In most cases, a celebration of the exodus--to be more precise, the refugees' survival as they fled the devastating Withering--is celebrated in some form during the year. Although there are many different versions of this celebration, each in a different part of the year, many people take part in others' versions of the celebration as well as their own.

In Literature

There are many works of literature in which the Withering and/or the exodus feature prominently. Some are written by outsiders inspired by the myth, but most are written by locals. There have been numerous fictional tales of characters caught up in the events, by authors coming up with their own ideas about what the Withering might have been or been like.
Only one piece of work survives from the actual generation who went through it: an epic poem written by an aristocrat from Jitua. It was missed in the purging of documents pertaining to the existence of the Five Realms which occurred some generations later, and by the time the mistake was discovered, it had already gained worldwide notoriety. Since most believed it was no more factual than any other poem based on mythology, it was left alone.