In J.R.R. Tolkien’s Middle-earth chronology, Númenor was the great island kingdom destroyed at the end of the Second Age.
Númenor was raised from the sea for the Men who had aided the Elves during the wars against Morgoth. Its people became renowned for long life, seafaring, and learning. Over time, however, fear of death and resentment toward the immortal Elves led many Númenóreans to challenge the limits set by the Valar.
Sauron was captured and taken to Númenor, but he corrupted King Ar-Pharazôn and encouraged an assault on the Undying Lands. Eru Ilúvatar intervened, changing the shape of the world and sinking the island. Elendil and his sons escaped with the Faithful and founded the realms of Gondor and Arnor.
Númenor is also called Westernesse. Its destruction explains the origin of the Dúnedain kingdoms and much of Aragorn’s ancestry in The Lord of the Rings.