Surash Ka, the Lurker in Shadow, is one of the Overlords who ruled over all of Eberron during the Age of Demons. Surash Ka was bound by the dragons and couatl after they jointly discovered the Draconic Prophecy, and the couatl sacrificed themselves to form the Silver Flame.
All of the Overlords embody mortal fears. The Lurker in Shadow embodies the fear of the unknown and the unknowable, of the terrible things that lurk just outside of sight and understanding. During the Age of Demons, the Lurker in Shadow claimed the Thunder Sea as its domain, and it was powerful enough that its influence is still felt by the Sahuagin and other aquatic peoples today. The sahuagin never speak its name aloud, referring to it only as the "Lurker" or the "Lurker in Shadow" if they need to refer to it at all.
The Lurker in Shadow has no representation in the Lords of Dust, and its favoured servants are Aboleths rather than Rakshasa.