Númenor is the island kingdom destroyed before the events of The Lord of the Rings.
In Tolkien’s mythology, Númenor is an island in the Great Sea, founded as a reward for the Men who helped the Elves during the wars against Morgoth. Its people receive unusually long lifespans, and the kingdom becomes a major maritime power.
Over time, many Númenóreans grow resentful of the Elves and fear death. Under King Ar-Pharazôn, the kingdom challenges the divine order and sails against Valinor. The island is then destroyed and submerged in a cataclysm known as the Downfall of Númenor.
Elendil and his sons, Isildur and Anárion, survive and establish the realms of Arnor and Gondor in Middle-earth. Númenor is therefore central to the ancestry of Aragorn and to the long historical background of Tolkien’s later novels.