From 1d4chan

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.

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.). The Doylist explanation is that GW writers started thinking it was strange that the elves had Khaine as a god of war & murder instead of worshiping Khorne for that stuff, yet worshipped Slaanesh for Slaanesh stuff. As such, they made up an elf god that promotes a Slaaneshi lifestyle in an attempt to make elf worship more consistent.

The High Elves and their gods of Warhammer Fantasy
High elf icon.jpg Characters: Eltharion - Everqueen - Teclis - Tyrion - Prince Imrik - Alith Anar High elf icon.jpg
Cadai: Asuryan - Hoeth - Isha - Kurnous - Lileath - Loec - Mathlann - Vaul
Cytharai: Addaioth - Anath Raema - Atharti - Drakira
Eldrazor - Ellinill - Ereth Khial - Estreuth - Hekarti
Hukon - Khaine - Ladrielle - Morai-Heg - Nethu
Events: The War of the Beard - The invasion of Naggaroth
Misc: Ulthuan - The Vortex - Waystone - Widowmaker
Appearances: Blood Bowl - Dreadfleet - Mordheim - Warhammer Fantasy Battle