From 1d4chan

Ellinill of the Cytharai is the Elf God of disasters and has more then a hundred different guises which to bring unpleasant ruin down on you and your family. Examples of his different forms for wrecking chaos and mayhem include Hukon the god of eathquakes, Addaoith the god of volcanoes and Estreuth the god of droughts.

The Dark Elves just love this guy and one of their favourite forms of worship to him is to chuck people off cliffs while singing his praises. They use him almost like a weapon, calling on him to constantly wreck the shit up for the High Elves. The High Elves in turn are constantly trying to appease him with offerings to ask him to go away so it really depends who pleases him more on a given week. The Wood Elves just tend to ignore him completely, which hasn't seemed to have harmed them.

Like with the other Cytharai, there is no mention of him in Warhammer 40k (although there are enough ongoing disasters to mean he is most likely lurking in the background or you know being digested by that bint).

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