Númenor was J.R.R. Tolkien’s invented island kingdom that sank in the Second Age.
The island was given to the Edain, Men who had aided the Elves during the wars of the First Age. Its people became the Númenóreans, renowned for long life, seafaring, and learning, and their kings later established realms in Middle-earth.
Númenor’s decline began when its rulers grew jealous of the immortality associated with the Elves and became increasingly hostile to the Ban imposed by the Valar. Sauron exploited this fear and persuaded King Ar-Pharazôn to attack the Undying Lands.
The resulting catastrophe destroyed Númenor and changed the shape of the world. Survivors led by Elendil founded Arnor and Gondor. Númenor is also called Westernesse in the Common Speech, but those names refer to the same kingdom.