There are multiple creation myths in the world of Tarosir, but one of them has the distinction of being based in fact. It also has the distinction of being mostly forgotten.
This is something of a work in progress, and I may edit the post periodically with revisions. On the other hand, I might make a new post if I come up with a revised version. It kinda depends on the extent of the revisions.
The Snake That Swallowed His Own Tail
Before the dawn of time, there was a colossal serpent. He was wise and benevolent, but also bored, for he was all of existence. So he decided to create a consort. He fashioned legs and wings for his consort, because he felt it would be more interesting if she were not the same as him. And then he breathed life into her. Her first act was to name them. She called herself Io, and her creator Cosmeon.
And life was interesting for a time, for Cosmeon and Io had each other to talk to. But eventually, they ran out of things to talk about. And then Io had an idea. Cosmeon had created her, after all; surely he could create many others to make life interesting!
Cosmeon thought on this, and realized that if he did as she suggested, they would need a larger place and more interesting place to live in. The infinite void did not seem like a good place for more than a few beings. And so, their conversations turned to discussing the world, to planning their creation. Finally, they were ready.
First, Cosmeon split the infinite void, forming the astral sea and the surrounding ring of chaos — the Astral Plane and the Far Realm. From this split arose the Plane of Dreams, like a ghost of the original void. Next, Cosmeon took a portion of his power and split it into its components, using it to plant the seeds of the Inner Planes. He positioned them so that their relative position could be used to navigate the Astral Plane; they defined up and down, north and south, east and west.
Cosmeon observed this for a little while, and realized that these planes were not an ideal place to live. So he combined a bit of each of the inner planes together to form the material planes. The Material Plane in which we all live, and the Faerie Plane; these he created and tied together with shadows.
And then he watched a little longer, and thought it might be good to have some guardians. So he created the mountain of Heaven to guard the forces of good, the pits of Hell to guard the forces of evil, the infinite clockwork machine of Mechanicus to guard the forces of law, the everchanging land of Errata to guard the forces of chaos.
And last of all, he constructed the world tree, Arborea, as a bastion of balance and neutrality, and built the paths of shadow, ether, and material substance that connect the planes together. Then he looked down on his work, and said, “This is good”.
Io watched him mold the world, and when he was finished, she spoke. “Now all it needs is people,” she said. Then she smiled and added, “And the first people will be my children.”
Then she gave birth to the first gods. Danube, the River of Life, emerged first, followed closely by Atuin, the World Turtle. Next was Morgoth, the Scourge of the Stars. Then Velka, the Spear of Light, and Tethys, the Star Traveller, emerged. Next to emerge was Bahamut, the Benevolent Flame, and she was closely followed by Azureus, the Searing Mind. Finally, last of all came Tiamat, the Burning Fang.
On Cosmeon’s orders, each of Io’s children went on to create the creatures they liked best. Danube created the animals of the land and sea, lakes and sky, mountains and rivers. Atuin created the sentient people of the sea, starting with the dragon turtles, but he also created other races, including the aboleth. Morgoth created the dark denizens of the underworld, and when he had finished his masterpiece, the dromites, they saw him for what he was and ran away. Velka created all the humanoid races, from the humans, dwarves, and elves to the orcs, ogres, and giants. Tethys created the homeless travellers who wander through the transitive planes, and when she had finished that, she created the fey races. Bahamut created the metallic dragons, Azureus the crystal dragons, and Tiamat the chromatic dragons. Io fashioned the remaining dragon types as well as the djinn and other elemental outsiders from the Inner Planes.
And Cosmeon himself fashioned the guardians, the people of the Outer Planes. In Heaven he placed noble races, with the angels to watch over them all from the top. In Hell he placed multiple warring races who could keep each other in check. In Mechanicus he placed the fomorians and the modrons. In Errata he threw the cast-off ideas he had considered for the other planes. And in Arborea he placed the impartial arbiters, the aeons.
And when he was done, Cosmeon laid himself out on the border of the chaos, at the edge of the Far Realms, and he put his tail in his mouth, using his own body to as a buffer between the chaotic madness outside and the astral sea within.