Númenor was the large island the Valar created for the Edain after the end of the First Age. Located in the western sea of Belegaer, Númenor was a seat of great arts, crafts, and learning for many generations.
While aiding the peoples of Middle-earth in the War of the Elves and Sauron, Númenor took the Dark Lord Sauron as a prisoner of war. The prisoner, Sauron, was a charming deceiver and soon became an adviser to King Ar-Pharazôn, luring the monarch and his people away from the worship of Eru (Ilúvatar). Sauron eventually persuaded the king that the secrets of immortality lay in the Undying Lands of Aman, the isle of the Valar that was forbidden to mortals.
In Second Age 3319, once the Númenórean armada landed in the Undying Lands, Eru intervened, creating a great cataclysm that removed Aman from the circles of the world and sank Númenor into the sea. During this cataclysm Arda was reshaped, islands sunk, new land surfaced, and rivers broke through or changed their course.