From what I can remember, and I might misremember some tiny stuffs; the chaos god Slaanesh was already kinda forming in the warp. At some point these space elves just got so bored with their lives, as they were very well off, they started to engage in these steadily worsening acts of sex, torture, mutilation etc; they even let it be done to themselves for, you know, a change of pace.
This sudden influx of chaotic energy caused by these deviant acts on an interstellar scale gave Slaanesh such a growthspurt that it instantly got pushed to creation, creating a violent warp/psychic explosion. As the space elves essentially “created” the chaos god they all suffered from this explosion, causing instant death. Only the elves in the webway where spared.
So actually, only a handful of Eldar as a race remain. I think the Dark Eldar learned their lesson now not to over indulge?
That must have been one heck of a warp eruption if the Eldar were considered like, or even greater than, the Imperium of Man in empire size for their entire civilisation to collapse in on itself from their hedonism
Only way I can try comprehend it is if all the humans of the Imperium where pyskers and were linked in real space and then having that link be chaos corrupted to such a degree that it fried their brains with the emotion made manifest.
I guess there must have been enough Eldar survivors that decided to not repeat that mistake and have it engraved in their ideology in pursuit of “purity” and those that saw the chaos, decided that it was meant to be, refused to change and figured out “loopholes” to not suffer the ill effects.
