Thereafter, Sméagol was forced into exile by his people, and creeping into the Misty Mountains, he began his transformation into the creature Gollum. The Ring quickly began corrupting him, and twisted his hobbit body and mind, and prolonged his life to an unnatural limit. He referred to the Ring as his "Precious," or his "Birthday Present."
He developed a dissociative identity disorder, going between his "good" personality, Sméagol, who vaguely remembered such things and friendship and love; and the "bad" one, Gollum, who sought to harm anyone who would try to take his Precious away from him. Samwise Gamgee would later nickname these personalities "Slinker" and "Stinker," respectively.
In July of TA 2941, during the Quest of Erebor, Bilbo stumbled upon the One Ring in Gollum's cave. The two took part in a game of Riddles, for which the Precious was the prize. Bilbo won, sending Gollum into a flying rage. Bilbo put on the Ring and learned of its power involving invisibility. He was able to get himself safely out of the caves and away from Gollum.
Frodo's kindness toward Gollum brought out the kind Sméagol personality; in Gollum, Frodo saw his own future if he did not destroy the Ring. He wanted to save Gollum so that he could also save himself.
When they reached the Black Gate, which was heavily guarded, Gollum convinced them to take another way into Mordor. Along the way, Frodo and Sam were captured by Faramir, and Gollum followed them. Gollum, feeling betrayed by his "Master," revealed his treacherous personality to Faramir when questioned about where he was leading the hobbits. When Faramir learned that Gollum was planning on leading them to Cirith Ungol, he warned Frodo and Sam.
Gollum's intention was to betray the two hobbits to the spider Shelob; however, a sleeping Frodo nearly caused him to change his mind and repent. But Sam woke up and began speaking harshly to Gollum, which then eliminated any hope of Gollum not going through with his plan. He led the hobbits to Shelob's lair, but Frodo and Sam escaped and made their way to Orodruin (Mount Doom). Again, Gollum followed them, hoping to surprise them. When he attacked, Sam fought back, but pitied the creature too much to go through with killing him.
Moments later, as Frodo stood at the Crack of Doom, he had a change of heart and put on the One Ring, unable to part with it. Gollum knocked Sam out and tracked down Frodo's footsteps. He jumped on the hobbit and wrestled with him before biting off his finger. Gollum, overjoyed at getting his "Precious" back, lost his balance and fell into the pit of the volcano, destroying both himself and the One Ring.
He was considered the fourth bearer of the Ring, following Sauron, Isildur, and then Déagol.
After Sauron, he was the second longest Ring-bearer.
His name in Westron was Trahald, which means "burrowing, worming in" or "apt to creep in a hole."