Former Hobbit. He was born in the Valley of Anduin, and once carried the name of Sméagol. In the year 2463 of the third age his cousin Déagol found a ring while fishing and was killed by Sméagol. The power of the ring (the one ring) prolonged Sméagol's life, but it crippled him, so he couldn't be regcognized anymore. He hated the light and lived deep under the earth where he nourished from carrion. Five centuries he lived in a cave. In the year 2941 the Hobbit Bilbo Baggins entered the cave and stole the ring from Gollum.
In the year 3019 Gollum was able to find the new ring carrier in Frodo Baggins. But only when they arrived at the end of the path Gollum attacked Frodo and bit the ring off his finger. In the moment of his triumph he fell backwards into the vulcan and was gone.