Isildur, son of Elendil the Tall, was a Dúnadan of Númenor. In his youth, he stole a fruit from Nimloth, and as a result, was able to preserve the line of the White Tree.
In the aftermath of the downfall of Númenor, he escaped to Pelargir, where, with his father and brother, he helped lead a remnant of the Dúnedain back to Middle-earth, and there they the founded kingdoms of Gondor and Arnor. Isildur became Lord of Ithilien and joint King of Gondor. He went on to found Minas Ithil, where he lived until Sauron took control of the city in SA 3429. Isildur then escaped to Arnor, leaving his brother to defend Osgiliath and Minas Anor, but returned later with an army of the Last Alliance.
In SA 3441, Isildur stood with his father in the final battle against Sauron, during which he cut the One Ring from Sauron’s finger, and, against the counsel of both Elrond and Círdan, kept it as retribution for the deaths of his kin.
|Harry Sinclair as Isildur in Peter Jackson's film adaptation|
Following Sauron’s defeat, Isildur ruled Gondor for two more years. He planted a seedling of the White Trees in Minas Anor in memory of his brother Anárion. He then travelled to Arnor, of which he had become King following his father’s death. On his way there, he and his men were ambushed by a host of Orcs in what was known as the Disaster of the Gladden Fields; in an attempt to escape, Isildur put on the One Ring and swam into the Anduin. The Ring betrayed him, slipping off his finger and revealing him to the Orcs, who slew him without hesitation. He was succeeded by his fourth son, Valandil, who took up Kingship of Arnor.