Númenor is the fictional island kingdom that J.R.R. Tolkien describes as being destroyed by the sea in The Silmarillion.
Númenor was created as a reward for the Men who aided the Elves during the wars against Morgoth. Its people received a long life and a great island kingdom in the sea between Middle-earth and Aman. Over time, fear of death and resentment toward the immortal Elves contributed to Númenor’s political and spiritual decline.
Under King Ar-Pharazôn, Númenor challenged the power of the Valar. Sauron was taken to the island as a prisoner but corrupted the king and encouraged the worship of Morgoth. Ar-Pharazôn eventually sailed against Aman. In response, Eru Ilúvatar changed the shape of the world and sank Númenor beneath the sea.
The survivors led by Elendil escaped and founded kingdoms in Middle-earth, including Arnor and Gondor. Númenor’s destruction is therefore both a tragedy and a major origin story for later events in The Lord of the Rings.