After briefly encountering Rangers of Gondor under the command of Boromir’s brother Faramir — who let them pass on to Mordor — Frodo, Sam, and Gollum made their way into Sauron’s dark realm. War erupted all around them, as the remainder of the Fellowship continued to battle Orcish armies in a bid to buy Frodo time to destroy the ring. Gollum, meanwhile, continued to plot the death of the Hobbits in order to retake the ring for himself.
In his bid to eliminate the Hobbits, Gollum led them to what he claimed was a shortcut to Mount Doom. In fact, he led them into the lair of the giant spider Shelob, who paralyzed Frodo with her venom. Sam killed Shelob and rescued Frodo, while Gollum seemingly vanished. Then, when Sam and Frodo finally reached the fires of Doom, Frodo gave in to the ring’s power and put it on, refusing to destroy it. It was here that Gollum made his move, pouncing on Frodo and biting off the finger that held the ring. Gollum, reunited with his precious at last, was too lost in celebration to see where he was standing. He fell off the edge of the Crack of Doom and into the magma below, destroying himself and the One Ring. Sauron, whose essence was directly tied to the ring, was defeated, and all of his armies instantly collapsed.
Frodo recovered from his injuries and the darkness of the ring, and sailed into the West with the last Elves of Middle-earth.