The Silmarillion
Akallabêth - The Downfall of Númenor
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At the end of the First Age, the Edain, who alone among the race of Men were loyal defenders of the Elves during their war against Morgoth, were given a new land of their own in reward for their efforts, free from the troubles and sadness of Middle-earth. It was located in the middle of the Great Sea, between the western shores of Middle-earth and the eastern shores of Aman, where the Valar lived.
The Men of Númenor were forbidden by the Valar to sail westward so far that they could no longer see the island, so the majority of their voyaging was eastward and in time they returned to the shores of Middle-Earth. The Númenóreans established remote colonies in Middle-earth and made contact with the Men of Middle-earth and taught them many crafts and skills. They made an alliance with Gil-galad and the Elves and aided them in the war against Sauron where he was defeated by the power of the Númenóreans.
Over time many began to speak openly of their envy of the Eldar's immortality and in defiance of the Ban. The people of Númenor became split between the King's Men, those loyal to the King and prone to oppose the restraints upon Númenor, and the Faithful, those who remained true to their friendship with the Eldar and loyalty to the Valar. During this time, the long life of the Númenóreans began to decrease as the Shadow rose. Their joy for life and goodwill departed, but their power increased.
Sauron re-emerged to challenge the power of Númenor in Middle-earth and their colonies along the shores were assailed by him and his armies. The king, Ar-Pharazôn, responded and came with a great host to Middle-earth and bade Sauron to come before him and swear fealty. To the surprise of many, Sauron did as he was asked. But the King was not content with his show of obedience and brought Sauron back to Númenor as a hostage. Sauron gave the impression that this was against his will, but in truth, it was exactly what he wanted. Sauron exploited his power to corrupt the King to his will. Soon he became his adviser, and most of the Númenóreans obeyed his will and turned to the worship of Morgoth.
Sauron convinced the King, who was now in his twilight years, to assail Aman in order to gain immortality. Thus Ar-Pharazôn led his Great Armament and landed on Aman. However, as this was done, the Valar appealed to Eru Ilúvatar and he destroyed the Great Armament. Ar-Pharazôn and his host were buried under hills, and the whole of Númenor sank under the Great Sea. Arda was made spherical and Aman was put beyond it, out of the reach of mortal Men.
Just a few people still uncorrupted by Sauron managed to escape the catastrophe; they fled Númenor by ship. This group of Faithful Númenóreans was led by Elendil the Tall and his two sons, Isildur and Anárion. They landed in Middle-earth, where the followers of Elendil established two kingdoms which came to be known as the realms of Exile: Gondor in the south, and Arnor in the north. Some of the King's Men, enemies of Elendil, who were in Middle-earth at the time of the Downfall established other realms in exile to the south; of these, the Haven of Umbar was the chief. Sauron, although greatly diminished and bereft of shape, had survived the Downfall and returned to Middle-earth to continue troubling its inhabitants.
Timeline of Arda - Second Age
1: Foundation of Mithlond the Grey Havens under Círdan, and Lindon as the Noldorin Kingdom under Gil-galad
32: Edain reach Númenor from Middle-earth, and Elros becomes the first King of Númenor
40: Many Dwarves abandon the ruined cities of Belegost and Nogrod in the Ered Luin and join Durin's folk in Moria
442: Tar-Amandil becomes the third king of Númenor
500: Sauron arises again in Middle-earth
600: First Númenórean ship sails to Middle-earth
740: Tar-Meneldur becomes the fifth king of Númenor
750: The Noldor found the realm of Eregion near Khazad-dûm
883: Tar-Aldarion becomes the sixth king of Númenor
1000: Sauron begins building Barad-dûr
1075: Tar-Ancalimë becomes the first ruling Queen and seventh monarch of Númenor.
1200: Sauron seduces and deceives the Noldor in Eregion
1280: Tar-Anárion becomes the eighth monarch of Númenor
1350: Celeborn and Galadriel together with their daughter Celebrían emigrate from Eregion to Lórien; Celebrimbor becomes lord of Eregion
1394: Tar-Súrion becomes the ninth monarch king of Númenor
1500: The Noldor under Celebrimbor are instructed by Sauron, beginning the forging of the Rings of Power
1566: Tar-Telperiën becomes the second ruling Queen and tenth monarch of Númenor
1590: The Three Rings are completed in Eregion
1600: Forging of the One Ring; Barad-dûr completed; Celebrimbor begins fighting Sauron; Glorfindel is sent back to Middle-earth by the Valar
1693: War of the Elves and Sauron begins, the Three Rings are hidden
1697: Eregion destroyed, Celebrimbor is killed.