Morgoth's Ring (HoMe 10), Laws and Customs of the Eldar, under the header "Of re-birth and other dooms of those that go to Mandos":
(This is a bit lengthy quote, but I think it's very informative as well)
Quote:
The fea [of an Elf who has died] is single, and in the last impregnable. It cannot be brought to Mandos. It is summoned; and the summons proceeds from just authority, and is imperative; yet it may be refused. Among those who refused the summons (or rather invitation) of the Valar to Aman in the first years of the Elves, refusal of the summons to Mandos and the Halls of Waiting is, the Eldar say, frequent. It was less frequent, however, in ancient days, while Morgoth was in Arda, or his servant Sauron after him; for then the fea unbodied would flee in terror of the Shadow to any refuge - unless it were already committed to the Darkness and passed then into its dominion. In like manner even of the Eldar some who had become corrupted refused the summons, and then had little power to resist the counter-summons of Morgoth.
But it would seem that in these after-days more and more of the Elves, be they of the Eldalie in origin or be they of other kinds, who linger in Middle-earth now refuse the summons of Mandos, and wander houseless in the world,* [A footnote: For only those who willingly go to Mandos may be re-born...] unwilling to leave it and unable to inhabit it, haunting trees or springs or hidden places that once they knew. Not all of these are kindly or unstained by the Shadow. Indeed the refusal of the summons is in itself a sign of a taint.
It is therefore a foolish and perilous thing, besides being a wrong deed forbidden justly by the appointed Rulers of Arda, if the Living seek to commune with the Unbodied, though the houseless may desire it, especially the most unworthy among them. For the Unbodied, wandering in the world, are those who at the least have refused the door of life and remain in regret and self-pity. Some are filled with bitterness, grievance, and envy. Some were enslaved by the Dark Lord and do his work still, though he himself is gone. They will not speak truth or wisdom. To call on them is folly. To attempt to master them and to make them servants of one's will is wickedness. Such practices are of Morgoth; and the necromancers are of the host of Sauron his servant.
Some say that the Houseless desire bodies, though they are not willing to seek them lawfully by submission to the judgement of Mandos. The wicked among them will take bodies, if they can, unlawfully. The peril of communing with them is, therefore, not only the peril of being deluded by fantasies or lies: there is peril also of destruction. For one of the hungry Houseless, if it is admitted to the friendship of the Living, may seek to eject the fea from its body; and in the contest for mastery the body may be gravely injured, even if it be not wrested from its rightful inhabitant. Or the Houseless may plead for shelter, and if it is admitted, then it will seek to enslave its host and use both his will and his body for its own purposes. It is said that Sauron did these things, and taught his followers how to achieve them.
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What is JRRT saying here?
* An Elf may refuse summons to Mandos and stay in Arda without a body
* Some of these dead Elven spirits are evil, servants of Morgoth
* Sauron and his followers' necromancy is about calling upon dead Elves
* The Houseless may wrestle the control of bodies of other beings
* Sauron taught his followers how to help the Houseless get bodies
I think that this explains the Barrow-wights perfectly. I also think that this also explains the werewolves. They were both evil Elven spirits with possessed bodies.
I think it's very unlikely that the Barrow-wights are Dwarves. First of all, Dwarves can't have been corrupted by Rings of Power. The Rings could only make them more greedy, but they could not be subjugated, turned into wraiths or be otherwise influenced. The Dwarves were also the most resistant race in Middle-earth.
The same impregnability which applies to Elvish and Mannish spirits probably applies to Dwarves:
nothing can force the spirit to escape its destiny. An Elf may be corrupted by evil, but evil can not
make the Elf to refuse the summons to Mandos. A Man might be spell-bound, evil and willing to cling to life, but after he has died
nothing can keep him in Arda against the will of Iluvatar.
It is of course possible that the Barrow-wights are minor Maiar, but I think that the Elf-idea is more likely.