Shade Perilous is the main focal point and stronghold of Nocturnal. It lies in Evergloam. It is one of seven realms Nocturnal possesses, as well was one of the realms that made up the Battlespire.
It is a place in which no light resides or shall enter. Supposedly, Ebonmere is the main way to enter and exit Nocturnal's realm, and it can only be unlocked and locked by use of the Skeleton Key.
Nocturnal and many lesser daedra reside here.