Slaanesh slew most of the Eldar and their Gods in the Immaterium, except for the Eldar God of War Kaela Mensha Khaine, whose energy was dispersed into many separate pieces scattered across the various Infinity Circuits of the Eldar Craftworlds, the Laughing God Cegorach, who fled into the Labyrinthine Dimension of the Webway, and while Isha was defeated, she was not destroyed outright and absorbed by Slaanesh like the rest of the Eldar Pantheon after his birth during the Fall of the Eldar.Slaanesh vanquished her as he had all of the other Eldar Gods within the Warp, but only took her prisoner rather than absorbing her energies outright.
What fell purpose Slaanesh had in keeping Isha alive, none amongst the Eldar now know, but the Prince of Pleasure was ultimately denied his spoils: for some reason Nurgle, the Plague Lord, waged war against Slaanesh to "rescue" the Eldar Goddess.
Why Grandfather Nurgle intervened is unclear, although some Eldar savants believe that the oldest of the major Chaos Gods wanted to give the youngest amongst them a good lesson about his proper place in the order of things.
That is how events are viewed from the chronology of the material universe.
In the Warp, things are different, for the Immaterium is not bound by linear four-dimensional time, and events do not occur in a strict sequence of cause and effect.
He is the God of Obsession, the Master of Excess in All Things, from gluttony to lust to megalomania.
Wherever mortals are ruled by their own unquenchable desires, the Dark Prince of Chaos is there in the shadows, whispering, tempting, and feasting on a banquet of souls.
The Fall of the Eldar in the early 30th Millennium was signalled by the birth-scream of Slaanesh, a tsunami of emotion that heralded the Prince of Pleasure arrival in the Realm of Chaos.
The psychic implosion caused by Slaanesh's birth swallowed hundreds of worlds at the heart of the Eldar empire in what is now the Imperium of Man's Segmentum Obscurus, killing billions of Eldar in a single instant and devouring a great section of the galaxy in the process.
Created by one species' pure dedication to indulgence, the first motes of what would become Slaanesh began to coalesce.
The dormant Slaanesh fed upon the unchecked collective psyche of the Eldar, drawing on their lusts and ambitions, their artistry and pursuit of excellence in all things.
Such was its ferocity that it overwhelmed the barrier between the material and the immaterial, forming the massive, permanent Warp rift later named by men as the Eye of Terror.