
Correctly assuming that his attempt to gain lordship had been thwarted, Sauron marshalled his armies to seize the Rings of Power by force. However, when he placed the Ring on his finger, the Elves were immediately aware of him. Soon afterwards, Sauron attempted to use it first to subjugate the Elven wielders of the other Rings. If it were damaged or destroyed, so too would be Sauron's strength and power. In this way, Sauron's fate became bound to that of the Ring. As such, he concentrated within the Ring a great part of his own fëa (soul). To accomplish his goal of bringing the peoples of Middle-earth under his dominion, Sauron knew that the One Ring would require an extraordinary amount of power. The Ring was forged around the year SA 1600, within Sammath Naur upon Mount Doom. As a result, Gollum’s transformation is down to the sheer length of time he spent as the guardian of the One Ring.Sauron forges the One Ring (by Ted Nasmith)

Every living character in the Lord Of The Rings can be corrupted by the Ring’s influence except, bizarrely, for Tom Bombadil. While Hobbits aren’t as susceptible as humans, they are still affected by the power of the One Ring.

This could explain why, much like Bilbo is not as monstrous as Gollum, the discernibly Hobbit-like Gollum is not as deformed and unrecognizable as the Ring Wraiths. In "The Shadow of the Past," Tolkien explains that Hobbits are less susceptible to the Ring’s powers than humans. This was a chilling reminder of the ring’s corrupting influence which, according to one wild Lord of the Rings fan theory, could have led Gollum to kill Frodo’s parents. He did, however, offer viewers a flash of his monstrous potential when he saw the One Ring in Frodo’s grasp. As a result, Bilbo did not become a monster like Gollum.

In contrast, Bilbo only maintained possession of the ring for 60 years, and he was a lot more careful about its corrupting influence. Gollum owned the One Ring for 500 years, during which time the powerful artifact turned him from an ordinary Hobbit-like creature into a monster.
