Secrets of the Universe
The Fall of the Old Ones
Note: This entire story came from a discussion between Gav Thorp, C.L. Werner, and myself after going over material from Bill King and Rick Priestly
To start, the Chaos Gods are a single entity. This entity is never given a name, though sometimes referred to as The Beast, The Dark Entity, The Source of All Evil (no! Not your mother-in-law!), etc. Regardless of whether 40k or Fantasy, The Beast is insane by human standards, being composed of at least 4 sections that constantly war with one another.
The Old Ones were stranded on the Warhammer World as most are aware. The Paths of the Old Ones mentioned in Bill's Giantslayer. Bill once had written a small essay on the Paths that he had posted on his site when it once still existed (it has since been taken down, much to everyone's chagrin). Essentially, they are only paths in the sense that the pipes of a nuclear power plant would be considered a pathway for rats. What the elves did when they created their vortex was essentially make a giant hole in the main storage tank of said pipeline.
When the Old Ones came to the world, there were only two intelligent races: The Dragons and the Dragon Ogres. Naturally, these did not bother the Old Ones none too much and they went about creating various species. Each race they created had a special part in their escape from the Warhammer World. Dwarfs could smith, Elves could wield magic, the Slann were designed for the purpose of watching for encroaching Chaos and to be an overseer to the others, Halflings were great cooks who could resist the effects of Chaos, and Ogres made great laborers/fighters who could resist the effect of Chaos.
There is one species though, that was not born of The Old Ones. That would be humanity. We are a creature of Chaos. Chaos designed us to be the downfall of the Old Ones. The only way they could get past the Slann's mathematical detection were in subtle ways. Thus, we were not Intelligently Designed. Not at all. We were evolved! Brutal natural selection was our only guide.
As the Old Ones prepared to leave the world, humanity gave the Dark Entity (or the Chaos Gods as a whole depending how you look at it) a much needed boost in power. They prayed en mass near the North in what are now the Chaos Wastes (the crown found in Malekith). This gave Chaos the ability to sabotage the Chaos Gates and cast the Old Ones into the warp wyrd/Realms of Chaos.
However, the Chaos Gods did not have the power to destroy the Old Ones. The Gods of Order are the shattered, broken, twisted, and warped remnants of the Old Ones (see Temple of the Serpent prologue). The fragments form the various gods as aspects of the Old Ones congregate and coalesce. There is only one unifying theme: They hate the Chaos Gods. They found fertile grounds in humanity as well. The humans who weren't in the North (i.e. not either turned into beastmen or madmen) took up worship just as quickly as they did of the Chaos Gods (and, ironically, the nature of Chaos is to try and destroy itself). The souls of humans who die feed the remnants of the Old Ones and allow them to continue their blighted existence. Without worshipers dying horrifically painful deaths "in the name of <god X>" said gods would not exist or have any power to influence the world around them.
Grim World of Warhammer indeed.