Isildur was a Dúnadan of Númenor, though he came to Middle-earth before the destruction of the island. He then founded Gondor in the South, became Lord of Ithilien and joint king of Minas Tirith, and also built Minas Ithil, and he dwelt there until its capture by Sauron. He left Gondor to seek aid against Sauron, and returned with the Last Alliance of Men and Elves. Isildur then fought beside his father until the coming fourth of Sauron, for his father was then slain by the Dark Lord. Isildur, taking up the shards of his father's sword, Narsil, cut off the finger of the Dark Lord and claimed the One Ring, destroying Sauron's body.

After the demise of Sauron Isildur became the King of Gondor for two years; until the year 2 of the Third Age. He taught his nephew Meneldil how to be a king, and then left Gondor in his hands, going north to claim Arnor. However, on the way the party with which he travelled was ambushed by Orcs in the Gladden Fields. Isildur put on the Ring to escape, but upon jumping into the Anduin to swim away the Ring became tangled in some reeds and fell from Isildur's hand, after which he was seen by Orcs and slain with arrows.