A map of Numenor
The island kingdom of the Dunedain, raised from the sea by the Valar as a gift and reward to the Men who had remained faithful through the dark years of the First Age. The Edain who had dwelt in Beleriand were led to the island in II 32 by Elros the Half-elven, who unlike his brother Elrond had chosen to be counted among Men rather than Elves.
Elros became the first King of Numenor, taking the name Tar-Minyatur. Under his rule, and the rule of his descendants, the Numenoreans rose to become the most powerful nation of Men in that or any other age. Their mighty ships returned to Middle-Earth in II 600, and there they founded havens and cities.
For the early part of their history, the Numenoreans were closely allied with the Elves of Tol Eressea, which lay close to their western shores. The Elves visited them often, and taught them much, but the Numenoreans themselves were forbidden to sail westwards, because the Valar feared they would become envious of the Undying Lands they and the Elves inhabited. As their greatness and power grew, the Numenoreans began to turn against the Ban of the Valar, and at last Ar-Adunakhor, who became king in II 2899, turned openly against it, though he did not dare defy it.
The last king of Numenor was Ar-Pharazon the Golden, who usurped the throne of the rightful queen Miriel in II 3255. He took his armies to Middle-Earth to make war upon Sauron, and so great had the Numenoreans become that Sauron's forces deserted him. Seeing an opportunity to destroy his enemy, Sauron sued for peace and returned with the King of Numenor. He gradually gained Ar-Pharazon's trust, and persuaded him to sail openly against the Valar. In II 3319 Ar-Pharazon sailed a vast armada into the West, but as he set foot on the forbidden shores of Aman, the Land of Gift was taken away and swallowed beneath the waves forever.
Some few survived the Downfall; Elendil, his sons and his followers had prepared themselves for the disaster and taken ship, and were driven back across the seas to Middle-Earth. There they founded the famous realms of Arnor and Gondor, though these were but a dim reflection of the glory of Numenor at its height.