Wraith-world

The Shadow World was a realm that the Nazgul were forced to inhabit, but also a world for the Calaquendi (Elves of the Light) to not live in, but to use its dark power. It appears as the ordinary physical world, though the world appears dark, gloomy, damp and distorted, because of the aura of the Ringwraiths.

If a Ring-bearer would wear the One Ring of Sauron, he/she would be shifted to the Shadow World; upon arriving in the spiritual realm, he/she appears completely invisible to the mortal and common people but very visible to the Ringwraiths (a common negative effect to the wearer).