Isildur was the king of both Gondor and Arnor. He fought with Gil-galad and his father, Elendil, against Sauron in the War of the Last Alliance.

He watched both Gil-galad and his father fall. However, he then used his father's broken sword, Narsil, to cut the One Ring from Sauron's hand.

Isildur then claimed the One Ring for his own and refused to destroy it. He was later ambushed by orcs and betrayed by the Ring. It slipped off his finger as he swam to escape. The orcs then shot him with arrows.

Frodo learns much of Isildur's history during the council at Rivendell. Here he also learns that Aragorn is the direct descendant of Isildur.