In the beginning there was Gaea, the force of nature. SHE filled the whole of existence with her being. After a few billion years, this became to grow boring and lonely, so SHE decided to have children.
At first SHE created a host of forces like Herself, eternal and divine. But with all their power, they couldn't get along with each other, so Gaea decided that wasn't a good idea. SHE decreed that from that point on all creatures but HER would have a limited lifetime. But most of HER first children disagreed, and went away to an unknown location.
So Gaea created a small world and populated it with limited creatures - animals and plants of all sorts. For a few million years she was happy to watch them. But her remaining faithful children were growing restless and wanted to make their own experimentations too. Finally SHE got bored again, as her creations weren't self-aware and therefore weren't much of a company. But SHE was also curious, because her creatures had developed many different styles of living, and SHE desired to compare them.
And Gaea destroyed the original world, breaking it in four new ones.
The First world was dedicated to harmony. It was populated with creatures that preferred to coexist peacefully and contemplate. Two of Her children were particularly enamoured of this world and decided to live there.
The Second world was dedicated to construction. It was populated with creatures that liked to change the environment around them, especially ants, bees and beavers, and filled with wood and metal and stone usable as raw material. Two of Her children found the idea fascinating and decided to live in this world.
The Third world was dedicated to war and strife. While all the three other worlds had equal shares of light and darkness, the Third world had as much light as its inhabitants desired - and it was populated with creatures that loved and hated light, in equal parts. So these creatures quickly discoreved they could influence the ambient light by battling each other. Two of Her children decided to live in this world: one because he loved light, and another because he hated his brother and therefore wished for darkness to prevail.
The Fourth world, finally, was dedicated to predation and to survival of the fittest. It was populated with fierce creatures and predators. Two of Her children believed this was the best way to live and decided to live in this world.
After the worlds were created, only four children of Gaea still preferred to live in the Void: two because they weren't interested in any of those worlds, and two because they liked all of them and preferred to visit them whenever they wished.
And for a few million years Gaea delighted in observing the differences between the worlds. But still the creatures were not self-aware, and still SHE was lonely. So when Her children came and said they were lonely and wanted to create more interesting creatures, SHE understood them and quickly allowed them and gave them the power to do it.
So on the First world, one of the Gods who lived there created a race of beings that loved peace and wildlife, and called them Elves. The other God in that world didn't choose to create followers, but she taught language to the Elves of her brother, and when the Elves learned the art of naming, they named their creator Lythander, and in exchange for his name he gifted them with luck.
On the Second world, one of the Gods created a race of builders and named them Dwarves. His children, in turn, named him Mostrai. His sister, in turn, decided not to create, but teach some of the dwarves (the ones willing to learn) about magic and the powers of the gods. The other dwarves, however, didn't accept their changed siblings, so they went off to live by themselves and became known as gnomes.
On the Third world, of course, none of the resident Gods wanted to cooperate with the other. Both created their own races: the God who fought for darkness created many kinds of fierce, brutal monsters he named “Demons”, while the God who fought for light created many kinds of skillful, swift warriors he named “Angels”. Befittingly, these Gods were named by their enemies, and not their followers: the demons gave the light god the name of “Valriel”, and the angels gave their enemy god the name “Gorokh”.
And on the Fourth world, the patron Gods kept true to their philosophy and created not one, but innumerable races, in the hope that one of these would prove the strongest and most worth. So they created goblins, trolls, ogres, and not least, dragons. The Gods became known as Gnarg, the Patron of Assassins, Ruggilli, the Consuming Worm, and Ixalovh.
The races in the four worlds lived on for millenia, and developed, and grew, and all worlds flourished. The eight patron gods believed this was proof that they were right, and the way they lived was the best one, while the two traveling gods rather believed that the success of all the worlds was instead proof of the wisdom and power of their Mother.
But at some point things started to go wrong. Angels and demons would start to become priests or scholars or merchants and preach against the war. Dwarves and gnomes would start go grow sedentary and admire the crafts of the past instead of building new ones. Goblinoids would suddenly develop a taste for vegetables and become farmers, while dragons would decide that developing their magics was more important than finding food. But what was wrong was a mystery that would only begin to unfold when the High King of the elves, one day, was devoured by a dragon.
It was then that the Gods discovered that the worlds were not anymore completely isolated from one another. Someone had, somehow, taught all kinds of creatures how to follow the paths left by the traveling gods - flying creatures would ride the cross-world whirlwinds of the thunder god, and others would learn to tread the strange winding paths of the walking god. And as creatures became aware of different ways, things started to go wrong.
Promptly, the Gods tried to close all portals - but it was too late. The art of magic was already known in the four worlds, and strong enough to travel between them. And the one peaceful elves were ignited, and declared war on the dragons, goblins and trolls.
While a mighty army of elven warriors crossed over to the Fourth world, dwarves and gnomes found the First world almost deserted and full of wood and metal for the dwarves, and holy, peaceful places for the gnomes.
The part of the small peoples who remained on the Second world was faced with new problems when the demons decided they were ripe for tempting (if they could get dwarven builders and gnome sorcerers on their side, the tide of the war would turn). The angels, of course, took upon themselves to protect these strange people from their eternal enemies. So as their attention slowly shifted from the Third world to the Second, soon only the weakest of their kind were in their original world, where they began to fall prey to hungry refugees from the Fourth world, who discovered how tasty angel/demon flesh can be.
And while the Gods were busy trying to even understand this chaos, the dead began to come back.
Their leader was Hagguth, the Elven High King who had been eaten by a dragon. He came back as a powerful spectre, leading a powerful army of ghosts, wraiths, skeletons and zombies in a massive attack to the Second and Third worlds. Their goal was to kill as many dwarves, gnomes, angels and demons as possible, to strengthen the number of their own undead host.
And when some elven wizards tried to steal the power of the undead to their own ends, they became the first vampires, lichs and demilichs - half living, half undead, but on the side of Hagguth and his mysterious masters.
It was only when a group of reasonable elves, dwarves, gnomes and dragons managed to get together and talk, that it became clear who these masters could be. Till this point, everyone was blaming some (any) god other than the one they worshipped. But there was only one, obvious answer: the Banished Gods, the Forgotten Children who left ages ago because they didn't agree with he concept of death, were back, and enacting revenge by subverting death itself. In later days, these Gods would become collectively known as the Devourers.
Knowing the enemy was not the solution to the war, but was a start. This group, slowly, was able to form a resistence against the undead armies, and gradually turn other people to their side. Finally, after 23 hundred years, they managed to break into the horrible fortress Hagguth had built in the First world and kill (again) the undead king.
The chaos began to gradually settle down after that. But then the survivors were faced with a new problem: almost nobody was interested in reverting to the way things used to be.
Yes. Entire villages of elves were settled in corners of the Fourth world for many generations, and dwarven guilds had business throughout the worlds.
So Gaea, in Her wisdom, gave the only possible solution to the problem. SHE took the four worlds in Her hand, and crushed them together. Then SHE looked at the single world she created, and at the creatures of Her children, and decided to again, after millions of years, create Her own followers. Thus SHE created the humans, a young race eager to explore and chart this young world.
It took generations for the inhabitants of the resulting world to figure out its geography and for families to reunite; when this finally happened, the stories about four different worlds becoming one were already legends not everybody believed. But still, for some reason, everyone agrees to call this world - the only one they know - Bigworld.
— Lalo Martins at some point in 2003 or whereabouts