[Thingol]

Heres the way I see it:

The men who were slain were placed in the Barrow Wights, and their fea (spirits) left Middle Earth forever. They the proceed to go to the halls of Illuvitar. The chief difference between the Nazgul and the Barrow Wights is that the Nazgul were never slain. There spirits never made it to the halls of Illuvitar because their life was unnaturally prolonged via their rings. The Barrow Wights on the other hand died before becoming Barrow Wights. It is not possible that the Witch King could bring back the spirits of men considering Manwe could not do it to Beren without asking Illuvitar. As for the Barrow Wights being inhabited by elf or orc spirits, this seems unlikely to me. Tolkien never mentioned an elf or orc Fea inhabiting another being in any of his works. I agree with what Man of the Wold said that orc or elf fea inhabiting the bodies of men does not sem consistent with the style of Tolkien's works. In my opinion it makes much more sense that the bodies of the men in the Barrow Wights were reanimated through the considerable power of the 9 rings. This is also consistent with the practice of necromancy. I've always thought of necromancy as the reanimation of the dead to create undead monsters (ex: skeletons and zombies). It is not the ressurection of the soul and the restoration of the body into its original form. Necromancy is merely the reanimation of a body; with the body being transformed into a hideous form and placed under the control of a powerful necromancer.