The Shadow Realm is a mysterious location in Gielinor. Although it seems to be part of the world itself, some believe it is, in fact, a different dimension.
The Shadow Realm is mentioned only in the Shadow Sword miniquest. During the miniquest, it is said that the Shadow Realm is a dimension in which the dead begin a new life, although the exact meaning of this is unclear. Whether or not all of the dead travel to this dimension is also unknown. The Realm itself contradicts many personal accounts and religious beliefs, and thus its nature (and, to some extent, its existence) is questionable.
Those that exist in the Shadow Realm may still wander Gielinor as spirits, invisible to all that do not have a Ring of Visibility. Some may also, it seems, physically interact with their surroundings.
Also the Shadow Sword and possibly Ghostly Robes are items from the Shadow Realm, as General Khazard talks to you if you only wear Ghostly robes, making him think that you are from the Shadow Realm (though he later reveals he knew who you were all along).