[Lord Halsar]

I do agree that the barrow-wights were not the spirits of men(I believe that it mentions somewhere that they were indeed simply spirits of darkness, or things that the Witchking had conjured at Carn-Dum, either way they were not the spirits of men). But I do think that the Morgul-Blades would make one more powerful than most of you(if not all) are describing.
I always imagined that by 'lesser darkness', it meant like how there were Maiar that were weaker than others, like comparing Sauron to Gandalf. In this case, we compare the Nazgul to the wraiths of their creation(or not?). Who knows? We never actually saw any of these wraiths (at least to our knowledge), and Frodo didn't die (or become possessed, however you want to call it) by his most greivous of wounds. The fact of the matter is, we really don't know. To the most of my knowledge, little was really said about these 'lesser wraiths' than what is said in the FotR. I simply wish Tolkien had said more of the matter, the topic in itself fascinates me to a great extent.