从imdb上第三部RotK的Trivia部分抄来的：
In the books, the prophecy that no man can kill the Witch King was originally done by the Elf Glorfindel, who foretold that "not by the hand of man shall he fall". However, the fact that Merry and Eowyn aren't human males is not the reason why they succeeded in the books: Merry had received an ancient Barrow-blade from Tom Bombadil (a character left out of the movie adaptations), an ancient sword specifically designed to be used against the Witch King's dark magic. It is that specific blade that Merry uses to stab the Witch King in the leg, and the book explains that "no other blade [..] would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will." Merry's attack therefore made the invincible Witch King temporarily vulnerable to Eowyn's attack, which happened to fulfill the symbolic prophecy that he wouldn't be killed by a man. In the film, Merry never received a Barrow-blade, and he uses a random sword, so in the movie, the prophecy is taken more literally, i.e. only someone other than a human male can kill the Witch King.
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FROM 216.240.30.*