The Lurker in Shadow

In the first age of the world, the Thunder Sea was the domain of a powerful overlord embodying the fear of the unknown and unknowable, of the unimaginable terrors lurking in the depths and in the darkness. Its true name is one more secret. Its servants call it Surash Ka, which is Abyssal for “the Deep Lord” or “the Ruler Below”; the sahuagin and other denizens of the Thunder Sea avoid even that name, calling it the Lurker in Shadow or just the Lurker. It’s an exceptionally powerful overlord; while unbound, it dominated the Thunder Sea, and even now, its influence can be felt across the region.

The Lurker in Shadow has no interaction with the Lords of Dust. Its servants include aboleths, shadow demons, and shark-aspected rakshasa, but stories say these are the least of the horrors it has spawned. While the Lurker has some overlap with Sul Khatesh and Tul Oreshka, Sul Khatesh is focused on arcane knowledge and personal secrets, while Tul Oreshka deals with secrets that can break people; the Lurker in Shadow deals with the things you can’t imagine, the forces that lie just beyond sight and that are waiting to pull you down.

Source: Chronicles of Eberron