During the Second Age, Sauron, Morgoth's greatest servant, had grown mighty in Middle-earth. Defeating all resistance and ruling through terror, force, and persuasion, Sauron dominated the peoples of Middle-earth, who looked to Númenor for aid. Heeding their desperate plight, the Númenorians dispatched an immense military force to Middle-earth, and after a period of heavy fighting, defeated Sauron and his hosts. Sauron hid within Mordor, where none dared to assail him, and re-built his strength. After once more reaching a height in his power, and with the armies of Númenor having left Middle-earth, Sauron was for a second time unopposed, and took on the titles Lord of the Earth and King of Men.
The Númenorians again received word of his rise in power. Angered by Sauron's pride and wishing to avoid another difficult war, the Númenorians swiftly came once more to Middle-earth with immense military force to confront Sauron a final time. So mighty were they that Sauron's forces fled from them with barely a fight. Sauron, however, realizing he could not defeat the Númenorians militarily, allowed himself to be captured, and begged, falsely, for mercy. The Númenorians brought him back as a prisoner, though through Sauron's persuasion, he rose from prisoner to advisor for the King himself. Convincing many officials to worship Morgoth, they quickly became corrupted to his side, to the point where Sauron successfully convinced King Ar-Pharazôn to betray the Valar and besiege them.
Elendil, son of the leader of the Faithful during the reign of Ar-Pharazôn, his sons and his followers had foreseen the disaster that was to befall Númenor, and they had set sail in nine ships before the island fell. His father Amandil the Lord of Andúnië, imitating Eärendil, decided to go West to warn the Valar about the King's plans.
Flooding of Númenor
Prompted by Sauron and fearing death and old age, Ar-Pharazôn built a great armada and set sail into the west to make war upon the Valar and seize the Undying Lands. Sauron remained behind. In the year 3319, Ar-Pharazôn and his Great Armament landed on Aman's shores and marched to the city of Valimar.
The Valar feared that the Númenorean army could wrack havoc in Valinor, but they were forbidden from killing or otherwise using force against Men. Manwë, chief of the Valar, thus called upon Ilúvatar, who destroyed the Great Armament of Ar-Pharazôn that had landed on Aman by crushing it under stones. He also broke and changed the world, taking Aman and Tol Eressëa from the world forever, changing Arda's shape from flat to round so that no sailor would seek the True West again. Númenor was covered by great waves and sank into the abyss, killing its inhabitants, including the body of Sauron; Sauron was robbed of his ability to assume fair and charming forms.
New lands formed in the West to replace the absence of Aman; islands rose and hills were formed. The coastline of Middle-earth also felt the effects of the cataclysm. In some places the coasts retreated, and in others they had advanced. Lindon in particular suffered great loss from the advance of the coasts, whereas the coasts had retreated to the east and south of the Bay of Belfalas, putting Pelargir much further inland than it had been and nearly destroying Tolfalas. As the Anduin found new courses along the new coasts, the Ethir Anduin formed.
Just a few people, the Faithful, still uncorrupted by Sauron managed to escape the catastrophe; they fled Númenor by ship, carrying some gifts that the Dúnedain had received from the Valar and the Elves in times of peace. These were the Palantíri, the Ring of Barahir, the Sceptre of Annúminas, Narsil and a fruit of Nimloth the Fair stolen by Isildur. They were led by Elendil the Tall and his two sons, Isildur and Anárion.
They landed in Middle-earth and met the colonists who were in the settlements of those lands among the Middle Men; the followers of Elendil united them by establishing two kingdoms which came to be known as as the realms of Exile: Gondor in the south, and Arnor in the north. The culture of Númenor became the dominant culture of Middle-earth (thus, Westron, a descendant of the Adûnaic language of Númenor, became the Common Speech). The sadness and the shock from the loss of their island home always remained in the hearts of Kings of Númenórean descent.
They were dismayed to discover that Sauron, although greatly diminished and bereft of shape, had survived the Downfall and returned to Middle-earth with the One Ring to continue troubling its inhabitants.
Meanwhile some of the King's Men had established other realms in exile to the south; of these the Haven of Umbar was the chief and continued to reign the lands there. These were known as Black Númenóreans.