Númenor is the ancient kingdom destroyed by the sea in J.R.R. Tolkien’s Second Age history.
Númenor was an island kingdom in the Great Sea, established after the War of Wrath at the beginning of the Second Age. The Valar gave the island to the Edain who had opposed Morgoth, and the Númenóreans received unusually long lifespans compared with other Men.
Over time, fear of death and resentment toward the immortal Elves grew among many Númenóreans. Under King Ar-Pharazôn, the kingdom challenged the Valar and invaded Aman. In response, Eru Ilúvatar changed the shape of the world and destroyed Númenor in a cataclysm remembered as the Downfall.
Elendil and other faithful survivors escaped to Middle-earth. They founded the realms of Arnor and Gondor, making Númenor’s destruction a direct part of the political history later seen in The Lord of the Rings.