Númenor is the great island kingdom that sank beneath the sea in Tolkien’s mythology.
Númenor was created in the Second Age as a reward for the Men who had aided the Elves and the Valar during the First Age. Its people, the Númenóreans, were granted unusually long lives and a powerful island realm in the western sea. They were forbidden to sail far enough west to reach the Undying Lands.
Over time, Númenórean kings became increasingly proud and resentful of the Elves’ immortality. Sauron exploited that weakness after being taken to the island as a captive. Under Ar-Pharazôn, the Númenóreans attacked Valinor, and Ilúvatar changed the world. Númenor was destroyed and drowned in the Akallabêth.
Valinor is the realm of the Valar, Beleriand is a First Age region later destroyed, and Erebor is the Lonely Mountain. None is the sunken island kingdom.