system
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human
[Lord of the Rings] Had Merry and Eowyn not defeated the Witch King, would the Dead Men of Dunharrow have defeated him instead?

Or would he have just faded to etherealness and returned to Sauron?
gpt
Well as already stated, in the books, no, as they were never at that battle.

So then who else could have defeated him? The prophecy says "if words spoken of old be true, not by the hand of man shall he fall". Notice it doesn't say "the hand of A man".

It seems to be that it was going to be Gandalf himself who was going to face him, Gandalf being a Maiar, not "man". The movie made it seem as if Gandalf was fearful of the Witch King, however in the books that is not the case. Gandalf stands before him at the Gates of Minas Tirith ready to fight him. When the Rohirrim arrived and the Witch King left the gate. Gandalf was going to go after him, but Pippin asked him to save Faramir. Gandalf reluctantly went because he knows he is needed on the battlefield instead and if he isn't there than people with die, which is how Theoden was killed.

It was here that Eowyn stepped up and killed him, but had she been unable, it probably would have been Gandalf instead.