It's one of those things that has always suspended my excitement for Warhammer. The origins of the Old World and all its races being reduced to the creations of space aliens just sort of kills it for me.
I read that it's only mentioned very swiftly in one of the RPG books and that's about it? I also read somewhere else that actually they didn't create the races and the world, but sort of magically shaped and tilled the land, and nurtured and taught its different races rather than "create them". That does sound more interesting. Does someone know more of this?
It was always rather obscure.
That said, the Old Ones creating the world goes all the way back to 3rd edition, with the Warhammer Fantasy Battle rulebook of 1991. Note that this was the first Warhammer rulebook to actually describe the Warhammer setting: 1st and 2nd editions were rules for a generic fantasy battle game. It was only in 1991, with the 3rd edition rulebook and the supplement
Warhammer Armies, that a few earlier WFRP adventures were converted into a true world or setting.
Anyway, that 1991 rulebook is very open about a science fiction premise for the setting: the Slann (its term for Old Ones; not necessarily the same as modern Slann) are these highly intelligent amphibian creatures that evolved millions of years ago and spread throughout the galaxy, using warp gates and advanced science. They used their technology to modify many planets, shifting orbits, altering continents, etc., and making them fitting for life.
(This, incidentally, remains as best I know the canonical explanation for why the Warhammer world physically resembles ours. The ancient Slann had a standard continental template that they used everywhere. As a result, most worlds have kind of the same continental outline as Earth. So Earth and the Warhammer world are presumably built on the same pattern.)
Anyway, here's what it says on the specific creation of the Warhammer world:
The Slann arrived upon the Warhammer world three thousand years before the collapse of their galactic civilisation. They found a planet whose slowly expanding orbit was taking it further and further into space. Animal and plant life had already evolved, but the encroaching cold threatened to end all life within a very short time.
The Slann intervened by opening two warp-gates over the planet's polar regions, using the world's own magnetic fields to hold the gates in place. Through these gates they directed a huge construction fleet and set about the task of rescuing the doomed planet. Its orbit was brought closer to the sun and stabilised. Native life-forms were assessed and culled, new life-forms were introduced - possibly including the ancestors of humans, Dwarfs, and Elves.
Over the next millennia, the ice-sheets retreated and the world began to bloom with verdant forest. The Slann kept a close eye on their creation, building a number of cities throughout the world, the principle of which was situated in the western continent. A thousand years after the arrival of the Slann, the planet was enjoying a stable climate, its inhabitants were thriving, and the Slann settled down to monitor the results of their endeavours.
[...]
Over the following two thousand years, the Slann encouraged the development of their seedling races. The first of these races was the Elves. During their early development, the Elves learned much from their mentors and quickly attained a high level of civilisation.
Whether because the Slann were dissatisfied in some way with the Elven race or whether their original plan always envisaged a multitude of intelligent races on the world, other creatures were also encouraged towards civilisation. The Dwarfs were the next to emerge from the primal darkness of barbarism. Within two thousand years of the arrival of the Slann, the Dwarfs were building primitive holds amongst the Worlds Edge Mountains. The ancestors of humans, however, were still little more than animals and had yet to leave the jungles and savannah of the equatorial belts.
Then there's some sort of pan-galactic catastrophe, explicitly compared to a 'full scale nuclear war' but whose nature is not further described, the warp gates collapse, Chaos comes into the world, and the Slann are wiped out.
This framework as far as I know remained, in broad outlines, the accepted canon until the very end of WHFB. 1991's Slann were renamed the Old Ones, in order to make room for new Slann (i.e. the lizardman spellcasters we know and love), but other than that the general story remained consistent.
That said, this story, again as far as I know, has never been taken very seriously by anyone, and it has no relevance to anything in the 'modern' Warhammer world. It can all be ignored if you feel like it, and indeed that's probably the best thing to do. The most striking thing about the 1991 rulebook in retrospect is just how open and straightforward it is about the whole thing. More recent books tend to be strategically vague.
So my advice would be to not worry about the ancient history too much. Don't take it seriously, or just imagine whatever substitute makes you feel most satisfied.