Let’s first see what exactly is the definition of immortality, within the lore of
Tolkien. We read in his Letters:
'immortality' (in my world only within the limited longevity of the Earth)
Let’s compare the two gifts to
Elves and
Men:
The doom of the Elves is to be immortal, to love the beauty of the world, to bring it to full flower with their gifts of delicacy and perfection, to last while it lasts, never leaving it even when 'slain', but returning.
The Doom (or the Gift) of Men is mortality, freedom from the circles of the world.
In other words, immortality is only confined among the circles of the world. Also, this immortality seems to me that refers to the “hroa” (body) and not the “fea” (spirit). We assume that the fea is immortal even after the end of Time (at least that is what the
elvish lore claims).
Let’s get to the
Ringwraiths now. Their fea was immortal by definition, without the interference of the rings. The immortality it gave to them, was bodily. In a way, it resembles of the elvish immortality. They did not age, but still they could be slain (
Witch-King). They lived in the Unseen, much like
Glorfindel being able to live in both the Seen and the Unseen. It seems that the 9 rings gave to the Ringwraiths elvish characteristics.
Longevity or counterfeit 'immortality' (true immortality is beyond Ea) is the chief bait of
Sauron – it leads the small to a
Gollum, and the great to a
Ringwraith.
But to what extent? And with which cost? Since this “transformation” was beyond the “laws”, it eventually became a burden to the “transformed”. Sauron could not change the nature of Men without side-effects. And besides, he didn’t care, as long as Men were lured.
Men and Elves (the firstborn) respectively, and could not be altered by anyone (even a Power or
god)
The “immortality” they got, I think was a deceit. After their death (which is certain to have happened with the destruction of the 9 rings), they did not stay on earth, like the Elves do, they did not get to a similar place like the
Halls of Mandos.
I am sure that they followed the fate of Men, whatever is their fate. Remember that no one can change their fate - except
the One and in very rare occasions (
Luthien,
Tuor).
What is this place they went, we can’t of course answer. But I don’t assume that this place is different for the good and the
evil. It may be a similar place like the Christian afterlife (waiting to be judged). But there can be no other possible fate for the Ringwraiths.
by Lhunithiliel
And in fact.... weren't they already just "
spirits" and not physically shaped humans when they lived as Sauron's servants under the spell of
the Ring?
Then, what happened to these spirits?
Spirits. We shouldn’t confuse the meanings we have in our world. They had still hroa and fea, just their hroa was not part of the Seen world. This is different from not having hroa at all, i.e. being just a fea that waits for its fate.
by Arvedui
IMO, this indicates that the Lord of the
Nazgul, at least, would be heard again at some later time.
Still, I do not consider this as decisive evidense. Your quotes more than indicate that their immortality is bound to
the One Ring.
I disagree with this explanation of the quote you provided. I find it just one of the linguistic tricks that the professor used to provide a epic tone in his writings. As I said before, I don’t think that the immortality of the Ringwraiths was authentic (like the elvish).