The Wraith-world or the unseen realm was an alternate dimension where cursed spirits such as the Ringwraiths exist.
The One Ring could draw any Ring-bearer to the wraith-world, allowing its denizens to be fully seen. It allowed the bearer to utilize strange powers and heightened olfactory and auditory senses. Any wraith within the realm would see the ring-bearer, and vice versa, and the bearer could be subjected to that world's weapons, such as the Morgul-blade. Additionally, upon wearing the Ring, the bearer would turn invisible except for the wraiths themselves.
Those who were in the Wraith-world could also see the brilliant, shining form of the High Elves known to exist in the other side of the threshold of the Unseen: Frodo Baggins saw Glorfindel in this form while wounded by the Morgul-blade.
Portrayal in adaptationsEdit
The Lord of the Rings film trilogyEdit
In Peter Jackson's Lord of the Rings film trilogy, the Wraith-world appears as the ordinary physical world, though the world appears dark, gloomy, damp and distorted, because of the aura of the Ringwraiths. Ring-bearers would be easily identified by the wraiths once he wears the Ring.
The Hobbit film trilogyEdit
In The Hobbit: The Battle of the Five Armies, the Elves of Mirkwood retain their white form once Bilbo Baggins, the Ring-bearer, wears the Ring and enters the Wraith-world(strangely this doesn't happen in The Hobbit: The desolation of Smauge).