I'd always wondered what would have become of Frodo if he had faded. Luckily - I guess - we never get to find out.
That said, in Rivendell we hear Gandalf talk about what 'could' have happened. How did Gandalf know? Where did he gain said knowledge? Was there any examples? Surely there must have been one, and that one human, as Gandalf compares Frodo to the yardstick of a human warrior (whom Frodo surpassed by many many days in bearing the splinter of Morgul).
I do agree that the notched "Wraithanator" knife born by the Witch-King was not a common artifact. I assume that this knife, and any like it, were the product of some spell that required some time and resources to prepare. So they weren't too common, but there must have been two - or another like item that created wraiths, as again the Wise have information about the effects of one. Why the King chose to use it against Frodo and not any Gondorian - like Faramir and Boromir - or elf or human king, I'll never know as it seems to me that he could have claimed the Ring with less expense, and used the 'hard to produce' knife for greater gain. A notched arrow may have been even more effective.
And note that I love the thought of a wraith Oliphaunt.
