- "They skin the body off you as soon as look at you, and leave you all cold in the dark on the other side"
- ― Gorbag
The Wraith-world was a mysterious alternative aspect of reality that belonged to the realm of the Unseen.
The Rings of Power were one of the means that could partly draw one into the Wraith-world. The Men who became the Ringwraiths, seem to have led a dual existence on the boundary between two worlds: our own, Seen world and the Wraith-world. After centuries they ended up fading and while they had a partial form in our reality, they had a more solid existence in the Unseen.
Their weapons known as Morgul blades also had the ability to draw others into their reality which seems to have been used to terrorize their servants. Frodo was wounded by one such enchanted weapon.
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.
Though Tolkien goes into very little detail about this 'other side' to reality, it is mentioned often in accounts that have come down to our own time through folklore. In these stories, it is commonplace to find fairies and elves - and less friendly beings - moving between our world and their own mysterious realm as parallel dimension. Indeed, these stories are, in part, the origin of the idea of 'fairyland', and it seems plausible that Tolkien's idea of the ghostly Wraith-world is in some way related to them. In the end, though, he gives us so little information that it is hard to do more than speculate the subject.