Atharti

Atharti of the Cytharai is the elf goddess of pleasure and indulgence, and so is doubtlessly popular with any right-thinking elf. She encourages all sorts of interesting and entertaining activities, orgies, drinking sessions, that sort of thing. The only reason she is a dark goddess is she encourages you to keep going regardless of what else you have to do, work, etc.

Occasionally she turns Dark Elves into Medusas.

You may be wondering why she is then worshipped by the Dark Elves but Slaanesh, who doubtlessly has a much broader appeal, isn't. It is because Atharti is 'official' and Slaanesh is banned. The worship of a member of the elven gods is perfectly fine with Malekith and helps to strengthen his rule, but the Chaos Gods, who demand loyalty from their worshipers, could undermine his authority; so he has banned Slaanesh and uses Atharti to fill in that hole (no joke intended you dirty minded people). It is for that reason the Dark Elves are still as depraved as ever, even with Slaanesh booted out of the picture (but not so many fun mutations like penis tongues, etc.).