"He will not return to this land. Far off yet is his doom, and not by the hand of man will he fall." —Glorfindel, The Return of the King, Appendix A (iv)
has been interpreted by many to mean "no man" could kill the Witch-king of Angmar.
I was thinking about the battle on Weathertop when Strider fought the Nazgûl with sword and fire to save Frodo. I'm pretty sure there were a few Nazgûl missing during the attack, and I'm not positive that the Witch-king himself was there, but it made me wonder if any of the other eight Wraiths could be destroyed by man? Strider, with all of his skills, didn't kill a single one.