Isildur (Quenya: [iˈsildur]) is a fictional character in J. R. R. Tolkien's Middle-earth, the elder son of Elendil, descended from Elros, the founder of the island Kingdom of Númenor. He fled with his father when the island was drowned, becoming in his turn King of Arnor and Gondor. He cut the Ring from Sauron's hand, but instead of destroying it, was influenced by its power and claimed it for his own. He was killed by orcs, and the Ring betrayed him by slipping off his finger just before his death. The Ring then became lost in the River Anduin. This set the stage for the Ring to pass to Gollum and then to Bilbo, as told in The Hobbit; that in turn provided the central theme, the quest to destroy the Ring, for The Lord of the Rings.
Tolkien began a time-travel story, The Lost Road, in which a father and a son were to reappear time and again in human families throughout history. One of the appearances was to be in Númenor just before its fall, with the father as Elendil and the son as Herendil, later called Isildur. The story was abandoned, but Tolkien reused the characters and events.