The Great Experiment

This world has very few myths regarding its origins, simply because the truth is plainly available to us. We call it the Overlord world because we have no better name for it. Names, after all, can be quite fleeting, can change as easily as signing a piece of paper. There may have been a name for it at one point, one all-encompassing word that meant “the entirety of the place where you now stand.” But it is long lost, and there are things much more important to care about. It has never much mattered to any person who is a native of this place, and only comes up in conversation at all when talking to a transplant. The only reason they think of it is they come from a place with a name, and a lack of something that seems so basic makes an impression. Also, I believe they find it difficult to address letters to home.
It is something shared by every person, of every race and every lineage, that we are all of us from another world. That all of us, somewhere down the line, were transplants.

Our Origins

The lack of specifics of technology and history indicates that there was not overmuch data to come with the travelers. So the initial shift to the Overlord world came at a time very early in the growth of the various cultures that now inhabit it. This is likely what enable the technology to develop at such an odd pace when compared with the one remaining example of the outside world we have left.
It is a common subject of the more poetically inclined historians to wonder what the first arrivals to our world might have thought. Were they frightened? In awe? Overcome by the beauty or by the danger? And why did they come? What would possess someone to unknowingly enter a portal and go to another world? Did they come here willingly or did they flee persecution and hardship?
Or perhaps they were simply coerced by those beings who created the portals, requiring as they did people to fuel their experiment. Those first portals were stable two-way gateways to the worlds they connected to. A trap, to allow those early settlers the belief that they were free to leave.
The history books state that the first nevus portal opened to Sornevo, the first gnomish portal opened to -dale, and the first elvish portal opened to Uncambel. All good, green countries with ample opportunity for things to grow and for a life to be built.
The first human portal opened to Denyithragorathia. The reason that they stayed is a mystery that has befuddled historians from a century after the arrival until this present day, because even in ancient times the country was hardly easy to live in. The legend, if such can be believed, is that it was a large group of them, frightened and uncertain. That it all nearly turned to violence and despair, then one man stepped forward and shouted his defiance to the sky, and that the sky shouted back at him. What was said is long lost, but whatever it was, that man turned and looked, and he saw beauty. And so humanity built itself a new home, hewn out of rock and danger and mystery.
With such a harsh homeland, it is not surprising that humanity was quicker to spread itself across the world, in search of places that were perhaps better conducive to an easy life. And that spread is perhaps also why more of them came through in quicker succession, to take the places of those who moved further in. The human population grew at an exponential rate that can also be ascribed to a higher rate of reproduction than any of the other three races.

The "Aliens"

I hesitate to call them such, since it would seem to give them greater mystery and grandeur than they entirely deserve, but they do deserve a mention. The reality is, all of those early settlers were lured, captured, or otherwise brought to this world by a species that exists beyond our stars.
We don't know much about them in the modern era because this world was something of a blank slate, with very little of their technology present. They created boundaries to their experiment in The Great Beyond, and they presumably had some kind of monitoring equipment in place.
None of this survived the uprising that turned against the aliens and drove them off. No one is entirely sure how. Wizards and sorceresses like to say it was magic overwhelming science. Scientists like to say it was early development of crucial technologies. Most other people tend to believe it was the overwhelming stubborn refusal to go along with the experiment being conducted that enabled us to throw off our chains.
We have no idea what the experiment was meant to achieve, nor do we know what has happened to the species that conducted it in the interim two thousand years since we drove them off.

The Portals

Approximately seventy years after the defiance, the historical record shows the portals disappearing. The nature of this disappearance is unclear. No major natural disasters were reported, no one writes about burying or hiding the portals, and no one says anything about the panic that would undoubtedly occur if the portals were to cease functioning. They simply vanish from the record, to the extent the precise sites of the portals cannot be located.
About a hundred years after this mysterious lack of information, a human sorceress named Velise created the first artificial portal using a magic ritual. The connection lasted for two days and resulted in sixty new humans added to the population of what eventually became Logenthru. After that, a number of individuals discovered various methods for the creation of stable portals. Travel between Earth and the Overlord world became, if not commonplace then certainly not an unusual and terrifying event.
No one has yet been able to connect with the elvish or gnomish worlds since the disappearance. The nevus records show a few portals to their own world, but their efforts drop off abruptly and in later years no connection could be made.

A Brief Note

As an aside, I shall make note of something scientists have marveled at for centuries. All our species, with fairly demanding requirements for survival, were able to come to this world, and not only were their diets and habitats compatible, but in the intervening centuries we have seen no sign of ecological degradation. In other words, our presence here has not caused a collapse of an ecosystem, which is in itself a crowning wonder of the world. I am, of course, not counting the various wars and waste dumping as little impact, but nor am I accounting for the effect of the various technologies and magics used to repair that damage.
And I sincerely doubt that the organizers of this great experiment were so clever as to have caused it by design.

Date of Setting

-300 to 70