tolkiens legendarium - Is it Éowyn or Merry who kills the Lord of the Nazgûl (or both)?

In the Lord of the Rings when the Lord of the Nazgûl is killed, there seems to be some confusion as to exactly who is responsible? Is the reason down to the sword of the Barrow Downs that Merry stabs him with, or does this simply make the Nazgûl vulnerable to normal weapons? The book seems to indicate that it is a combination of the sword strikes from Éowyn and Merry and that both were necessary for his demise (that is just my interpretation of how things are written). The prophecy that he cannot be killed by the hand of mortal man is not much help in explaining this since neither Éowyn nor Merry is a man!

Answer

There's no doubt that Eowyn's was the killing blow. There is however some debate about the exact role played by Merry - and, in particular, Merry's sword. Here's the key passage:

So passed the sword of the Barrow-downs, work of Westernesse. But glad would he have been to know its fate who wrought it slowly long ago in the North-kingdom when the Dûnedain were young, and chief among their foes was the dread realm of Angmar and its sorcerer king. No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will.

This seems to hint that it was the sword, not the wielder, which undid the spell that made the Witch-King invulnerable. But it also makes it clear that that blow was key to enabling Eowyn's killing stroke. So, would a Man have been able to strike that same blow, if he had happened to be there holding the same sword?

I'm inclined to think that the question is moot. The prophecy was not "no man can kill me", it was "not by the hand of man will he fall": Glorfindel, making the prophecy, was just foreseeing that it would be not be a man who would wield the sword. And both Merry and Eowyn fit that description.