Who Really Killed the Witch King in ROTK??

[Gordis]

Firstly I want to state that I have not a slightest doubt that the nazgul had physical bodies, albeit invisible.

Secondly, if we go speculating, I also have a theory. I believe the WK had been pierced by a blade before, but not necessarily by a Barrow-blade. I think the major difference between ordinary blades and BD blades is that the former disintegrate at once, at the first contact with nazgul blood - so they simply can't do much damage. The enspelled blades do disintegrate, but later, after the damage is done.

Speculation on my part about how the knives – both Barrow-blade and Morgul-knife – might operate. They may have no similar function (besides, this is all fantasy magic!), but they do seem to be mirrors of one another, do they not? The Morgul-knife drew Frodo into the shadow-world. The Barrow-blade made the Witch-king vulnerable to Éowyn’s blow.

But Frodo had been vulnerable to the nazgul blow before - as was any living thing. So the situation is different, IMO