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Ancient home of the Dúnedain
The island kingdom of the Dúnedain, 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 Númenor, taking the name Tar-Minyatur. Under his rule, and the rule of his descendants, the Númenóreans 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 Númenóreans were closely allied with the Elves of Tol Eressëa, which lay close to their western shores. The Elves visited them often, and taught them much, but the Númenóreans 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 Númenóreans began to turn against the Ban of the Valar, and at last Ar-Adûnakhôr, who became King in II 2899 turned openly against it, though he did not dare defy it.
The last King of Númenor was Ar-Pharazôn the Golden, who usurped the throne of the rightful queen Míriel in II 3255. He took his armies to Middle-earth to make war upon Sauron, and so great had the Númenóreans become that Sauron's forces deserted him. Seeing an opportunity to destroy his enemy, Sauron sued for peace and returned with the King to Númenor. He gradually gained Ar-Pharazôn's trust, and persuaded him to sail openly against the Valar. This he did in II 3319, 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 Númenor at its height
Source : Encyclopedia of Arda , the best friend of a Tolkien newbie
Hope this is helpful. If you have any other questions, feel free to ask.