User blog comment:Drayco90/The Witch-King vs. Perseus/@comment-4178963-20110810123721/@comment-143604-20110811164727

It's not a spell and nothing was broken. The Witch-King was told that he would not by the hands of mortal man die. It wasn't any special ability, it was a brief glimpse into the future and the Witch-King took it as an invincibility. And either way is irrelevant because Perseus is a demi-god, not a man.