That line reffers only to the Witch-king, once it was killed by Eowyn
.
Edit: it is worth noting that in the "Ring goes south" chapter, Gandalf says:
Quote:
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You cannot destroy Ringwraiths like that, said Gandalf. The power of their master is in them, and they stand or fall by him.
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which could imply their dissappearence (either as impotence, either all mortals depart).