system
Given the following post, respond as if you're commenting on Reddit:
human
[LotR] What are the powers of the One Ring?

Besides making the wearer invisible (and why does it do that anyways?), what exactly does the ring do? Boramir, Denethor, and countless others wanted to use the ring to defeat Sauron, but what advantage would it give them, assuming they could resist its corruption?
gpt
The One Ring enhanced Sauron's power, especially as it pertained to controlling the wills of others. Those who have their own Rings of Power (ex. Elrond, Galadriel, and Gandalf) would have been especially vulnerable to someone who mastered the One Ring, but even those with no connections to the Ring aren't immune. It's the kind of power that allowed Sauron go from a prisoner of Numenor to the high priest and top adviser to the king. Even Sam gets access to a tiny fraction of this:

> His will was too weak and slow to restrain his hand. It dragged at the chain and clutched the Ring. But Sam did not put it on; for even as he clasped it to his breast, an orc came clattering down. Leaping out of a dark opening at the right, it ran towards him. It was no more than six paces from him when, lifting its head, it saw him; and Sam could hear its gasping breath and see the glare in its bloodshot eyes. It stopped short aghast. For what it saw was not a small frightened hobbit trying to hold a steady sword: it saw a great silent shape, cloaked in a grey shadow, looming against the wavering light behind; in one hand it held a sword, the very light of which was a bitter pain, the other was clutched at its breast, but held concealed some nameless menace of power and doom.

> For a moment the orc crouched, and then with a hideous yelp of fear it turned and fled back as it had come. Never was any dog more heartened when its enemy turned tail than Sam at this unexpected flight. With a shout he gave chase.

But mastery of the Ring would allow much greater powers of control and persuasion of others' wills.

Perhaps even more significantly, mastering the Ring would defeat Sauron, just as if the Ring had been destroyed. If its power were mastered by another, that power would no longer be sustaining Sauron, and he would be reduced to a shapeless spirit.

Long life and invisibility are side-effects of the Ring, originating from its inherent power and inherent corruption, respectively. The "invisibility" is more like being drawn into an unseen world. The Ringwraiths exist entirely in the unseen world, and so are invisible to anyone not wearing a Ring of Power. Especially powerful/divine beings, such as Glorfindel, Galadriel, or Sauron, exist in the seen and unseen worlds simultaneously (and would not become invisible upon wearing the One Ring).

Though it's also extraordinarily unlikely Boromir or Denethor could actually have mastered the One Ring; doing so requires immense power and force of will. Most people have no chance at mastering the Ring, but the Ring itself deludes people into thinking they can.