Elendil was one of the faithful Númenorians who founded the kingdoms of Arnor and Gondor after Númenor (also called Westernesse) was sunk beneath the sea (see bookmark for page 316). Elendil was High King of both kingdoms, choosing to rule from Arnor while his sons Isildur and Anárion ruled Gondor.
Together, Gil-galad and Elendil formed the Last Alliance of Men and Elves in order to destroy Sauron and put an end to his evil. The vast army they amassed consisted of elves from several realms including Elrond of Rivendell, men of Arnor and Gondor, and some dwarves. During the battle, both Gil-galad and Elendil were slain, but Isildur picked up the hilt-shard of his father’s sword Narsil and cut the Ring from Sauron’s hand. Sauron’s physical form was destroyed and the battle was won. The Kingship passed to Isildur, who was killed by orcs soon after. The Ring fell into the Gladden River and was lost (eventually making its way to Sméagol/Gollum).