- "And here in Rivendell there live still some of his chief foes: the Elven-wise, lords of the Eldar from beyond the furthest seas. They do not fear the Ringwraiths, for those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power."
- ― The Fellowship of the Ring, Many Meetings
The Unseen are objects and creatures of the spirit world that belong to 'magic'. Some of the most obvious examples of Unseen creatures are the Ringwraiths, which were invisible and formless without the guises provided by Sauron.
The One Ring shifts its wearer to the wraith world who then could see the forms of other persons. Also, The Morgul blade that hit Frodo, had the ability to transform him into a wraith and permanently bring him to the Unseen realm; while Frodo was suffering from its effect, he could see Glorfindel's true luminous form.
The mysterious individual known as Tom Bombadil could see Frodo wearing the Ring; whether this implies that he lived in both realms is not clear.
Portrayal in adaptations
2001: Peter Jackson's The Fellowship of the Ring: The scenes where Frodo wears the Ring, show a different, ghastly version of reality, indicating that Frodo entered another realm, although no explanation is given in dialogue. The effects of the Ring become more clear in the Weathertop scene where he sees the Ringwraits in visible form while wearing the Ring.
In the book, Frodo under the effect of the Morgul blade sees the luminous form of Glorfindel, but since in the movie he is replaced by Arwen, Frodo is supposed to see the Unseen version of Arwen; her appearance to Frodo is in a white dress bathed in an aura of light.