Further to previous answers:
The Witch-King of Angmar is/was (depending on when your chronicle is set) unable to be slain by a blade wielded by man. This would imply that it had been tried to my mind. And, there are a distinct lack of other wraiths around. Of course, death by a Morgul Blade takes one into the realm of shadow. Whether or not such a weapon needs to be wielded by a 'wraith is unknown to me. I would assume not, but...
(Just my take on things)
Cheers
Tas
I'm NOT stupid, I'm NOT expendable and I'm NOT going!