Unseen

The Unseen are objects and creatures of the spirit world that belong to "magic". Some Maiar like Olórin were said to walk unseen among the Elves of Valinor, and Ossë would visit the shores unseen. All things in Arda fall into the realm of either the Seen or the Unseen. The Unseen world is neither good nor evil and contains both dark and bright spiritual beings.
== Nature ==
The Valar could exist in the world with or without a corporeal form, "the Valar may walk, if they will, unclad, and then even the Elves cannot clearly perceive them, though they be present." Wizards and the Elves who lived in Valinor existed in both the Seen and the Unseen realm; their form in the Unseen is different, and they have the ability to see and affect Unseen creatures.
The One Ring shifts its wearer to the "wraith-world", in which the wearer could see the forms of other persons. While the wearer would seem invisible, in reality, they would be visible to the creatures of the Unseen realm. Also, the Morgul-knife that stabbed Frodo had the ability to permanently bring him to the Unseen realm.
The Unseen world also existed within the Halls of Mandos. There the spirits of those who died in Middle-earth would reside until they met with their different fates. The Eldar may, in time, be re-embodied, if they chose to be as Finrod did, and live again in Aman. The Dwarves would wait in Mandos, in separate halls among their own, until the re-making of Arda. And Men would reside there for a time of waiting before receiving their fate which brought them beyond the scope of Arda, until the Second Music of the Ainur.
Spirits which did not heed the call of Mandos may remain in Middle-earth as part of the Unseen. Though the Witch-king, as a bearer of a Ring of Power, summoned the barrow-wights to the Barrow-downs, it is doubtful that he could have called spirits back from Mandos' halls.
== Wraith-world ==
The Wraith-world was a mysterious alternative aspect of reality that belonged to the realm of the Unseen. Some of the most obvious examples of Unseen creatures were the Ringwraiths, which were invisible and formless without the guises provided by Sauron.
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 blades seem to have been used to terrorize their servants, as Gorbag attested to, and if the broken shard of one such blade had reached Frodo's heart, he would have been transformed into not only an Unseen existence, but into the wraith servant of a Nazgûl.