The story of the WHFB world starts in an unusual place, it starts on Fenris. A laboratory world of the Old Ones, where a certain hairless primate species was experimented on to create a weapon against Chaos. The Old Ones drew upon human legends and sought to create a culture resistant to Chaos and strong enough to fight them. Thus the Fenrisians and their harsh world were born. Eventually wider humanity discovered this and was outraged. The Old Ones apologized and gave ownership to humanity in exchange for something. Gene-data on humans and abhumans. A deal was struck and this sect of Old Ones disappeared into a system "bottled" in the Webway. In this system, they created a world modeled on Old Earth and set to work. Dropping two "control samples" on the world, baseline Humans and the legendarily hardy Zoats. (One of the eldest creations of the Old Ones, and among the few that spread to other galaxies.)
Using the Slann species as laborers, the Old Ones set to work experimenting with variants of humans. Splicing of humans and Aeldari became the Elves. A refined version of the Squats became the Dwarves. An upgraded but unfinished version of humanity became the Ogres. The bottled system came with another advantage. It cut off part of the Warp from Chaos touch and let these experimental species develop new gods unaffected by the Warps' corruption. However, two forms of contamination made it into the experiment. The ever-persistent Orks, and the whispers of Chaos touching the mind of a young brash Old One.
Arrogant and clever, this Old One had been denied the right to create a species he'd designed to fight Chaos. His seniors saw numerous flaws in the project, which relied on concepts that Inquisitors in the 41st millennium would call extremely Radical. Chaos saw an opportunity and took it. Corrupting this Old One, and guiding him to betray his colleagues. So when the time came, the Warp gates on the experimental world were breached with his aid, and the Old One's artificial moon-ship of Blackstone was corrupted into Warpstone. Nearly all the Old Ones of the sect were devoured or fled. Only the traitor and a spawnling hidden by the Slann, named Kroak, survived.
The traitor got his reward, ascending to Daemonhood. Becoming a Daemon King and God to his designed species. Which he unleashed in a rain of Warpstone. Now the traitor took a new form and name. The only reminder of his previous state as a reptilian Old One is his horns. As such the Great Horned Rat, and the Skaven were born. While the spawnling bided his time, his egg a twin-tailed comet orbiting the world. Only becoming visible when a new crack appeared in its shell. Eventually, the spawnling awoke, still growing in his egg, but powerful and furious at the betrayal. He reached out to a Skink and gave him a mission. Thus the Cult of Sotek was born and the Slann species declared war on their arch-traitor.
Eventually when the influence of Chaos became too great and the laboratory world buckled under the stress. The egg hatched and the last of the Old Ones took flight in a form modeled after the greatest of his people's creations, the Dragon. Finding a ruined world, with a battered God-born-mortal clinging to it. The two of them, the only successes in an experiment that ended tragically, escape into the abandoned world-forges of the Old Ones. Where they might combat the threat of Chaos in the bottled universes of the Webways depths.