Númenor was the island kingdom given to men by the Valar for the part they played in helping the elves in the war against Morgoth. Elros, brother of Elrond and son of Eärendil, was chosen as its first king. The Númenorians became known as the Dúnedain to the elves.
The Númenorians were at first a great race, enlightened by the wisdom of the elves. However, they eventually began to grow resentful of their mortality, and of the ban placed upon them against ever visiting the Undying Lands. They grew aggressive and once more entered Middle-earth, beginning to form a large empire. The last king of Númenor, Ar-Pharazôn the Golden, defeated Morgoth’s old servant Sauron, captured him and brought him back to Númenor. Here Sauron became the king’s counsellor and began to persuade him to take immortality by force. Ar-Pharazôn listened, and built a huge force to assault the Undying Lands.
As soon as the Númenorians reached the shores of the Undying Lands, the Valar called upon the power of Ilúvatar. The kingdom of Númenor was destroyed and swallowed by the sea. Some loyal Númenorians survived, and sailed to Middle-earth to found the realms-in-exile of Arnor and Gondor.