Númenor is the fictional island kingdom that sank in Tolkien’s legendarium.
Númenor was an island realm created by the Valar for the Edain after the defeat of Morgoth at the end of the First Age. Its people, the Númenóreans, were granted long life and great knowledge. The island lay between Middle-earth and Aman, but its inhabitants were forbidden to sail west to the Undying Lands.
Over time, Númenórean kings became increasingly concerned with death and immortality. Sauron exploited those fears and was taken to the island as a prisoner, where he corrupted King Ar-Pharazôn and encouraged rebellion against the Ban of the Valar. Ar-Pharazôn eventually led an armada toward Aman.
The resulting catastrophe destroyed Númenor and changed the shape of the world. Elendil and his sons survived and founded the kingdoms of Arnor and Gondor in Middle-earth. Tolkien’s account of Númenor is told most fully in “Akallabêth,” part of The Silmarillion.