Númenor is the realm destroyed in the Downfall of Númenor in J.R.R. Tolkien’s legendarium. The island kingdom was founded in the Second Age for the Men who had aided the Elves during the wars against Morgoth. Its people received exceptionally long lives and settled on an island west of Middle-earth.
Over time, many Númenóreans became resentful of the Elves and fearful of death. Under King Ar-Pharazôn, the kingdom challenged Sauron, who was captured and taken to Númenor. Sauron then corrupted the king and encouraged the Númenóreans to attack the Undying Lands in an attempt to obtain immortality.
The Valar responded by changing the shape of the world and sinking Númenor beneath the sea. Elendil and his sons escaped with faithful followers and founded the kingdoms of Arnor and Gondor in Middle-earth. The story is told most fully in “Akallabêth,” a section of The Silmarillion.