Númenor was the great island kingdom of Men in J.R.R. Tolkien’s legendarium before its downfall.
The Valar raised Númenor from the sea at the beginning of the Second Age as a reward for the Men who had fought Morgoth in the First Age. Its people, the Númenóreans, received unusually long lifespans and became skilled sailors. Elros, the half-elven brother of Elrond, became the first king of Númenor.
Over time, many Númenóreans grew resentful of their mortality and became increasingly ambitious. The kingdom’s later rulers sought power over other peoples and eventually challenged the will of the Valar. Ar-Pharazôn, the last king, led a fleet against the Undying Lands, and the island was destroyed in the resulting catastrophe.
Survivors led by Elendil founded the kingdoms of Gondor and Arnor in Middle-earth. Númenor is therefore central to the ancestry of Aragorn and the political history of the Third Age.