system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Are there any stories relating to the origins of the Chaos gods other than Slaanesh?

Slaanesh has a pretty fleshed out backstory relating to the Eldar while Khorne, Tzeentch and Nurgle just sort of "are". Papa Nurgle is said to be the oldest, so there has to be at least some sense of their beginning. 

I've also seen it said that Slaanesh is distinctly Eldar in nature and thirst for souls. Do the other gods have racial tie-ins as well?
gpt
I've always been under the impression that the other 3 gods had always existed in one way or another, due to representing 3 universal constants: Decay, Death, and Change. Stuff has been doing those three things constantly forever, so they've always had a powerbase to build on.

But Slaanesh was different. It wasn't till the height of the Eldar Empire that decadence was practiced on that scale. Sure, everything fucks but that's not really what Slaanesh is about. It's about excess, and it took an entire Empire's worth of murderboning to make him all at once, rather than gradually growing stronger like the others. That's why she tore open the Eye of Terror, rather than just forming in the Warp.