Gollum, originally known as Sméagol (T.A. c. 2430 – 25 March 3019), is a fictional character from J. R. R. Tolkien's legendarium. He was first introduced in the author's fantasy novel The Hobbit, and later became an important supporting character in its sequel, The Lord of the Rings.
Gollum was a Stoor Hobbit of the River-folk, who lived near the Gladden Fields. Originally known as Sméagol, he was later named Gollum after his habit of making "a horrible swallowing noise in his throat". His life was extended far beyond its natural limits by the effects of possessing the One Ring, and he became obsessed with the Ring that had enslaved him, to the point that he pursued the Ring for the rest of his life after losing it to Bilbo Baggins.
During his centuries under the Ring's influence, he developed a split personality: "Sméagol" still vaguely remembered things like friendship and love, while "Gollum" was a slave to the Ring who knew only treachery and violence. In The Two Towers, Samwise Gamgee named the good personality "Slinker", and the bad personality "Stinker". The two personalities often quarrelled when Gollum talked to himself (as Tolkien puts it in The Hobbit, "through never having anyone else to speak to") and had a love/hate relationship.
Gollum is first introduced in The Hobbit, where the protagonist Bilbo Baggins stumbled upon Gollum's lair and found the Ring which Gollum had lost in the network of caves leading to the lake.
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Links: Wikipedia: Gollum The Thain's Book Lordoftherings.net: Gollum