system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings] How do the Nazgul/Ringwraiths work?

I know they're undead, but past that, I don't know what's going on there. Their bodies are gone, but they're corporeal somehow or something? Can someone explain that one to me.
gpt
There is the world, and the unseen world. The Maiar seem to be native to the Unseen World, but can fashion bodies to appear in the regular world. Sauron is cursed to always seem evil to men in corporeal form.

Presence in Unseen World can be equated to magical power. Looking upon the Two Trees gave the Eldar a presence in the Unseen World, and this is the source of their "glow." Men don't have have any native presence in the Unseen World.

The Rings of Power have the effect of bringing the wearer into the Unseen World. This makes them invisible to persons in the regular world and allows them powers they don't normally have. They see and can be seen by other beings in the Unseen World. Glorfindel could not join the Fellowship of the Ring because he had an exceptionally strong presence in the Unseen World and would be trivially easy for the Ringwraiths to detect.

The Ringwraiths were kings of Men who wore rings subject to the One Ring. They gained a presence in the Unseen World, and thereby power. They also became subject to Sauron's will, as Sauron provided the rings.