Ereth Khial

Ereth Khial, the Pale Queen is the High Elf and Eldar goddess of the dead. She is second only in power to Asuryan (which means she is stronger then the war god Khaine! Scary thought).

Warhammer Fantasy

A very powerful goddess who once tried to suck Asuryan's dick while he was passed out on the couch watching Blood Bowl reruns. He woke up banned her from ever entering his house again, which pissed her right the fuck off. But because she can't do anything to him personally, she decided to take it out on mortal elves.

She established her own realm seperate from the divine plane of the elves.

While the favored servants of the elf gods dwell in their respective domains, and kings and high nobility live with Asuryan, mortal elves of low class and no renown who would normally dwell on Ulthuan through the Waystones forever are drug by her servants, banshee-like creatures called rephallim, to her hell-like underworld where they are tortured constantly. While sometimes Slaanesh gets to snag an elf soul that dies too far from a Waystone and who's patron god wasn't paying attention at the time, he has to compete with Ereth Khial for them. Generally speaking, as an evil god, she's not worshiped. However those elves too afraid of being eaten by Slaanesh and thus ceasing to exist offer her prayers in hopes she'll get them first. Society frowns on this greatly, and there's a stigma that only the truly insane worship her (at least amoungst the High Elves and Wood Elves).

The Dark Elves worship her and the rest of the Cytharai openly and she is honoured by her mortal subjects. It is a good thing too because it is said if she were ever angered like she was when Asuryan spurred her advances then she would raise the dead and unleash them on the living.

40k Version

Since it's stated that no Eldar gods survived the birth of Slaanesh, it's likely Ereth Khial was raped and eaten. This may be what gave Slaanesh the ability to claim the souls of the Eldar, since he's more or less biting off of Ereth's style in doing so. Otherwise, it's possible that Ereth survived since she may have occupied an entirely different plane of existence than the rest of the Eldar gods, and may be laying low and waiting for Ynnead to finish Slaanesh off to take control of the now ownerless Eldar souls. Furthermore, it's possible that Ynnead IS Ereth Khial, and that she sent a vision to the Eldar race of her plan to become a Chaos God herself. Wait a minute, that doesn't sound grimdark? Oh it does. Remember that being nommed by Slaanesh is preferable to what Ereth Khial does to you (then agian, the High Elves are a bit more honorable than the Eldar who tend to run and hide when they aren't manipulating shorter lived races into doing their dirty work so Slaanesh's belly may be the honorable afterlife seppuku to being an Eldar).