Q: What Could Sauron Have Done Had He Regained the One Ring?
ANSWER: Everything that the Elves (and Gandalf) had accomplished with the Three Rings would have become known to Sauron. He would have known who learned what through use of the Three, who healed what or whom, and where the Rings were hidden. At the council in Rivendell, Elrond disclosed the following to Gloin and everyone else attending:
…Those who made them did not desire strength or domination or hoarded wealth, but understanding, making, and healing, to preserve all things unstained. These things the Elves of Middle-earth have in some measure gained, though with sorrow. But all that has been wrought by those who wield the Three will turn to their undoing, and their minds and hearts will become revealed to Sauron, if he regains the One….
Sauron originally made the One Ring to enhance his ability to influence and suppress or control the wills of other beings, specifically the Children of Ilúvatar (Elves, Dwarves, and Men). Because the One Ring was the master to all the Rings of Power, it would have given Sauron an easy means of dominating or enslaving the wills of the Keepers of the other Rings.
Although the Dwarves never became Ringwraiths or slaves to their Rings, the way the nine Men who accepted Rings of Power did, Tolkien suggests that the Elven keepers would not have been able to resist Sauron’s will as well as the Dwarves. Ultimately in time they would have fallen under Sauron’s control, and through them he would have gained control over many Elves.
I think it doubtful that all Elves would have been enslaved if only because many Elves lived outside of Rivendell and Lothlorien. Whether Gandalf would have succumbed is anyone’s guess. He would not have been able to use Narya to defy Sauron, if Sauron regained the One Ring. But I think that Elrond, Galadriel, and Gandalf would have had the opportunity to remove their rings — as Celebrimbor and his contemporaries did when Sauron first forged the One Ring — and maybe that would have been sufficient for Gandalf to fight off the subsequent influence of the Ring.
Unlike the Three, Sauron seized the Seven and the Nine during the War of the Elves and Sauron in the Second Age. He took the rings back to Mordor and “perverted” them (as the narrative says). By saying that Sauron perverted the rings Tolkien apparently means that Sauron altered their natures in some way. For example, it was clearly never the Elves’ intention that the original keepers of the Nine should become Ringwraiths. The Seven and the Nine conferred the ability to see Unseen things, and to render Unseen visible things (presumably the wearers themselves).
Tolkien’s essay concerning Elvish spirits seems to explain the reason for why the Elves made the Rings in the first place. They were trying to forestall the inevitable fading to which they were doomed (if they remained in Middle-earth). By delaying the effects of Time the Elves slowed their natural aging to about 1/10th of the normal rate. Hence, a thousand years for an Elf under the influence of the Great Rings was really more like 100 years — and Elves hardly aged at all in the course of 100 years, so they would not have to worry about fading for a long time.
But some Elves may already have faded by the time the Gwaith-i-Mirdain made the Rings. Or, it may be that they desired to converse with the spirits of Elves who had died in Middle-earth and who had rejected the summons of Mandos. When Melkor (Morgoth) was still self-incarnated during the First Age, according to Tolkien, he could force any elvish spirit to reside with him in Angband if the dead Elf refused Mandos’ summons. But what became of those trapped spirits after Melkor was defeated?
And in the Second Age, what happened to the Elvish spirits who refused the summons? Tolkien implies they would have become “haunts” (ghosts), perilous for Men to interact with, but desiring (eventually) to return to their own bodies or to seize the bodies of others. Hence, necromancy would have become a dangerous form of sorcery for Men but might actually have been one of the goals of the Ring-makers.
Sauron therefore may have been able to use the One Ring to seize control over elvish spirits — at least after the Elves became subject to the power of the Three Rings. So whereas in the Second Age, before the Three were used much, the Elves could flee Middle-earth, they might have been forced to remain had Sauron recovered the One Ring. Such a fate might explain why so many Elves fled Middle-earth at the end of the Third Age. The Great Exodus (as I call it) is briefly described in “The Shadow of the Past” in The Fellowship of the Ring, where the narrative mentions that Elves are seen passing through the Shire on their way to the havens, never to return.
The fear that drove the Elves to abandon Middle-earth must have been very great indeed. They saw no hope of defeating Sauron, but they could not simply all take ship and carry the Three Rings over Sea — not while they were still functioning and subject to the will of Sauron embedded in the One Ring.
So I think that ultimately Sauron would have been better able to seize control over the minds and spirits of both Elves and Men. Dwarves, perhaps, might have been able to resist but in some late writings Tolkien suggests that many Dwarves had by this time fallen into evil. They may have been more vulnerable to Sauron’s domination than the “good” Dwarves like Durin’s Folk.
Hence, with the Power of the One Ring to strengthen him, Sauron would have been invincible. No one would have been able to raise an army to defy him. Even without the One Ring he was able to master or dominate the wills of many tens of thousands of Men and Orcs at the end of the Third Age. They were left in disarray when Sauron heard Frodo claim the Ring for himself, understanding at last what his enemies’ plan was. Were he reunited with the One Ring, Sauron would have been able to corrupt nearly all of Middle-earth the way he had corrupted the Númenoreans at the end of the Second Age.