Interesting. On the one hand, Tolkien wrote in the Athrabeth: 'The Elves believed, though they had no certain information, that the fëar of Men, if disembodied, left Time (sooner or later), and never returned", with the following note added: "Sooner or later: because the Elves believed that the fëar of dead Men also went to Mandos (without choice in the matter; their free will with regard to death was taken away). There they waited until they were surrendered to Eru. The truth of this is not asserted."Arthadan wrote:Men spirits, unlike Eleves spirits which may linger if they are corrupted, must answer the Summon of Mandos and go to the Halls of Mandos. From there, they leave Arda for good, nobody knows their final destination save Eru. Only known exceptions are the Nazgûl due to the use of Rings of Power which weren't meant to be worn by mortals and the Oathbreakers who were cursed by a Maiar-related bloodline (Elendil, scion of Melian) using the sacred name of Eru himself as witness. For the record, Barrow-wights are "evil spirits" inhabitating the deceased bodies of kings of old.
So, no disembodied human souls in Middle-earth since willy-nilly they all went to Mandos (or so the Elves believed).
On the other hand, the Oathbreakers are a pretty big exception. If disembodied spirits can be stopped from going to Mandos at such a scale, through the agency of a mere mortal (regardless of progeniture), then any argument that there can be no ghosts in Middle-earth seems pretty shaky. Surely there must be other oathbreakers, and possibly other means than broken oaths.
Fairë in Quenya meant "phantom; disembodied spirit, when seen as a pale shape". If the Barrow-wights and the Gibbet King are 'evil spirits', how do we know that they were not disembodied once?