Númenor was the fictional island realm where Tolkien’s Númenóreans lived before it sank.
Númenor was created as a kingdom for Men who had aided the Elves during the wars against Morgoth in the First Age. The island lay in the Great Sea between Middle-earth and Aman. Its people enjoyed long life, great skill, and prosperity, but they were forbidden to sail west to the Undying Lands.
Over time, fear of death and resentment toward the immortal Elves grew among many Númenóreans. King Ar-Pharazôn eventually challenged Sauron and was persuaded to attack Aman. Ilúvatar then changed the shape of the world: Númenor was destroyed and removed beneath the sea, while Aman was taken from the ordinary world.
Elendil and his sons Isildur and Anárion escaped the disaster and founded the kingdoms of Arnor and Gondor in Middle-earth. Númenor is therefore the ancestral source of the Dúnedain and the later royal lines central to The Lord of the Rings.