: The Encyclopedia of Arda writes that the Seven and the Nine
: themselves had the power of invisibility of the wearer but also
: a corrupting power of trapping the wearer in the "Wrait-world".
: Here's what Arda have to say about the Great Rings:

[snip]

Some of what he writes is correct, some of it is incorrect. The Rings did not bestow or extend life. They were created for the purpose of holding back Time, but also had other powers. The Nine and the Seven "conferred invisibility", as Tolkien put it, but they did so by somehow associating the wearer's physical body with the wraith world, which is a separate aspect of Ëa, the universe. There is a physical and a non-physical aspect to the universe in Tolkien. They coexist, together, and are closely tied together. I'm not saying that every rock or plant exists in both aspects. Rather, anything with a "spirit" or "soul" exists in both aspects, but only SOME spirits are directly aware of the non-physical wraith world. These include the Valar and Maiar, and some Elves (Gandalf says specifically that those who have lived in Aman dwell in both worlds at the same time -- this is what I mean by "are directly aware").

The lengthening of the lifespans of Men and Hobbits ("mortals") by the Rings was an unforeseen side-effect of their powers.

Sauron/Annatar's objective with the Rings of Power was to enslave the Elves. Some people have inferred that the Elves might eventually have become wraiths even if Sauron had not made the One Ring and subsequently captured and perverted the Nine and the Seven, but I'm not so sure of that. The Elves were trying to prevent or delay their inevitable fading. They were already doomed to become wraiths eventually, if they elected not to sail over Sea. So the Rings were -- from their perspective -- the only means they had of NOT becoming wraiths.

Whether Sauron could have altered the Rings substantially after he captured them is debatable. But Tolkien says he captured them, took them back to Mordor, and there perverted them before giving them out to Dwarves and Men. It was his intention to create Dwarven Ringwraiths, but Aule's design frustrated his efforts, and the Dwarven Rings failed to turn their owners into wraiths. The Rings did, however, apparently induce their keepers to become greedier and perhaps more paranoid.

Other powers that the Nine and Seven possessed included "rendering visible the unseen" (apparently this means the Elves wanted to be able to make wraiths appear visible to living eyes), and enhancing the natural powers of the wearers. The Three did not confer invisibility, but they did help to hold back Time, and Elrond said that all the Rings were intended to help with healing, making, and understanding. So the Three would have possessed these abilities, and they were apparently more powerful than the Seven and the Nine.

The ability to render visible the unseen wraiths of the world would have made the Ring-keepers powerful necromancers. I don't doubt that this is what Tolkien was referring to (in part) when he said the Nazgul became "kings, sorcerors, and warriors of old" when they were first given the Rings. Their sorcery probably included other powers, but at the very least they should have been able to render themselves invisible (by wearing the Rings), to render wraiths visible to others (probably by wearing the Rings and exerting their wills -- and perhaps this means they would eventually have been able to make themselves visible), and to dominate the wills of others (a power Sauron may have wanted his slaves to have, in order to extend his own power).

With these powers alone, the nine Men who received the Rings could have become exactly what Tolkien said: kings, sorcerors, and warriors of old.
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