Going back to the origin of the
Barrow-blades, I would also add this to Inder's convincing list of quotes:
The Lord of the Rings: The Return of the King, "The Battle of the Pelennor Fields"
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
Dunedain were young, and chief among their foes was the dread
realm of
Angmar and its sorcerer king.
If you look at the bold writing, apart from all the other quotes Inder has provided us with, it is clear that the sword was a work of Westernesse.
Anyway, about the immortality of the
Witch-king, this is my opinion -
I think that we are looking at the whole "immortality" thing the wrong way. The
Ringwraiths were not "immortal" as
Elves were. They survived while
the Ring survived.
They were
wraiths, that is, they were of the spirit world, and were only made visible when they wore clothing - otherwise they were completely invisible (save the eyes) to anyone
Hobbit,
Dwarf or Man who lived in the normal world.
They were wraiths because they became enslaved to their nine Rings - just as
Gandalf explains to
Frodo in the chapter "The Shadow of the Past"; if one wears a
Ring of Power too much you begin to "fade", and eventually you will become invisible to any other who lives soully in the normal world. You will become a wraith. Not a
Ringwraith, but just a wraith, "lesser, and under their control".
So basically, as the Ringwraiths were enslaved to their Rings, and in turn
the nine Rings were under the power of
the One Ring, the Ringwraiths survived as long as the One Ring did.
Therfore, the
Nazgul were not immortal. They were sort of "stuck there".
When
Eowyn and
Merry combined to defeat the Witch-king, they erased him from the normal world - he could not grow or take shape again. He was completely powerless. But he was not completely "gone" or "dead" until the One Ring was destroyed.