Now we all know about the prophecy that "no living man" may slay the Witch-king of Angmar in LOTR. This prophecy was fulfilled when Éowyn, a female, stabbed the Witch-king in the head, thus killing him.
But what if instead of Éowyn, it was a Man, like say Aragon or Éomer, who delivered the "fatal" blow to the Witch-king with their sword? Would the prophecy hold true and the Witch-king NOT die?
For one thing "Prophecies" in Tolkien's work aren't the "Nostradamus" kind of prophecies that we are used to, it's not a fortune teller saying "you'll meet a tall dark witch-king and end him". Neither is it a fairy tale curse or enchantment Before your 16th summer you shall prick your finger . . . there by forcing fates hand.
These are not a Tolkien prophesy.
Tolkien was more of a "fate" guy, for example when the elves gave chase to Morgoth after the kinslaying they were given the Doom of Mandos. That isn't "a punishment" but lays out the consequences foreseen by those who can see further and more clearly the effects of a given action. If you do this, then this will be your fate.
Glorfindels words that became "The Witch-king prophesy" are . . .
He will not return to this land. Far off yet is his doom, and not by the hand of man will he fall.
... and were spoken to Eärnur, a "man" ergo, non elf. So Glorfindel may have been saying "This foe is beyond your powers, it would take an Elven Lord (like me) to bring him down, and I can't see myself taking him on anytime soon".
On the other hand, it may have been Glorfindel getting a sense of the Witch-king's doom, he may have felt that it wasn't a man who would kill him.
Also the magic that protected the Witch-king wasn't specific to "men" (i.e. this shield does a quick chromosomal check of the attacker and just deflects blows from people with Y chromosome). The magical protection was magical and protected him against all blows.
However while Eowyn landed the fatal blow, it only succeeded because Merry's first blow unwound the Witch-king's magic defenses.
To answer the question
But what if instead of Eowyn, it was a Man like say Aragon or Eomer who delivered the fatal blow to the Witch-king with their sword? Would the prophecy hold true and the Witch-king NOT die?
If it had been Aragon or Eomer who landed the blow after Merry disabled the Witch-king's shields (as it were), then the Witch-king would have died, and proven the "prophecy" false, however that didn't happen. Could it have happened? Let's let Morpheus answer that
No, what happened, happened and couldn't have happened any other way.
rec.arts.sf.written covering this point; the bit I remember is "No man can slay me" "I am a woman" "Bah, this Westron is so imprecise, I didn't mean vir, I meant homo" "In that case, permit me to point out that Meriadoc, who is not homo but [some made-up Latin word], a Halfling, has just introduced a blade of Gondolin to your knee." Sadly, DejaNews is no longer a thing.
No, the thing is, the prophecy that the Witch-king would not be slayed by a man was just that: a prophecy. It was a prediction of the future. So, what happened was, Glorfindel probably looked into the Witch-king's future and saw that no man would kill him.
This looking into future business is finicky, unless you use Galadriel's mirror. With the mirror, you can actually see events that happen in the future, past or present: "Things that were, things that are, and some things that have not yet come to pass". But even then, it's not set in stone. Things in the future may change. Same with Elrond (and later, Arwen) who actually saw Arwen having a son with Aragorn, though the future is not set in stone.
But with Glorfindel, and all these 'set in stone' prophecies (like the Words of the Seer about the Doors of the Dead), they can't physically see what's gonna happen, they just get a feeling, or they don't find what they're looking for. Like with Glorfindel: he tried to figure the Witch-king out, and couldn't find the future where a man kills him. So that's what he prophesied.
It's not that because of the prophecy, no man will be able to kill the Witch-king, more like no man is going to kill him, therefore the prophecy exists. In other words, a situation where a man stabs him or tries to kill him in any way, simply was never meant to happen.
The case of Huan is similar, and instructive, because someone did try to cheat. Huan was a wolfhound, and there was a prophecy saying that he could only be killed by "the greatest wolf that would ever live". Sauron (yes, that Sauron) turned himself into the greatest wolf that had ever lived, attacked Huan — and was defeated. Some years later, Huan was slain by a still mightier wolf, Carcharoth, who had not yet been born when Huan fought Sauron.
I don't recall exactly how it's put, and don't have a copy of The Silmarillion to check, but I believe it is made reasonably clear that Sauron could not qualify as "the greatest wolf that would ever live" because that was Carcharoth. That position was already taken.
So with the Witch-King: he must have fought plenty of "men" (however we define that) in the course of his career as a warlord, conqueror, and Ringwraith, but it was Eowyn and Meriadoc who killed him, and Glorfindel predicted that "not by the hand of man will he fall" because it was Eowyn and Meriadoc who killed him. In the scientific terms familiar to us here in the Fifth Age, the causality runs backward in time; if he had fallen in some other way, Glorfindel would have seen something different.
(One can interpret Ainulindalë to be saying that the entire history of Arda is laid out in the Music of the Ainur, and therefore any event is knowable in advance, although only Ilúvatar knows all of it.)
In addition to other answers here, I'd like to point out that Eowyn was able to kill the Witchking thanks to Merry stabbing him in the knee with a barrow-blade, distracting him with the surprise of pain and breaking the spell binding his undead flesh to his will. Eowyn would have probably been slain then and there if it were not for Merry.
Her shield was shivered in many pieces, and her arm was broken; she stumbled to her knees. He bent over her like a cloud, and his eyes glittered; he raised his mace to kill. But suddenly he too stumbled forward with a cry of bitter pain, and his stroke went wide, driving into the ground. Merry’s sword had stabbed him from behind, shearing through the black mantle, and passing up beneath the hauberk had pierced the sinew behind his mighty knee.
- The Battle of the Pellenor Fileds, LotR
This brings us to the fact that Eowyn was able to carry Merry with her on her horse only because she was a woman and didn't weigh as much as a man. Theoden told Merry previously that none of his Riders can bear him as a burden.
Thus, in that strange way that prophecies find their fulfillment, the Witchking couldn't be killed by a man, because it needed a Hobbit and a woman for the job :)
