Númenor is the island kingdom destroyed in J.R.R. Tolkien’s legendarium. It was located in the Great Sea between Middle-earth and Aman and was granted to the Men who had aided the Elves during the First Age.
The Númenóreans received a long life and great knowledge, but they were forbidden to sail west far enough to see the Undying Lands. Over generations, fear of death and resentment toward the immortal Elves grew among many Númenóreans. Their king, Ar-Pharazôn, eventually challenged the Valar and sailed against Aman.
The Valar appealed to Eru Ilúvatar, who changed the shape of the world. Númenor was overwhelmed and submerged, while Aman was removed from the ordinary world. The event is remembered as the Downfall of Númenor.
Survivors led by Elendil escaped and founded the realms of Arnor and Gondor in Middle-earth. Númenor is therefore linked directly to the ancestry of Aragorn and the history of Gondor.