I see it as quite possible that Glorfindel's "prophecy" came to the Witch-King's attention and that he misinterpreted it as a condition rather than an instance of foresight.
Perhaps he extracted it from Earnur under torture. Assuming that he captured the unfortunate fellow rather than dispatching him outright, it is quite possible that he would have been interested to learn what the Free Peoples knew of him.
Hmm. A rather unpleasant thought occurs. Bearing in mind what the Witch-King told Eowyn that he had in mind for her, it seems to me that Earnur might have met quite a horrific end indeed ...
