The Downfall of Númenor, also called the Fall or Drowning of Númenor, was the utter destruction of the island of Númenor, a divine punishment by Eru Ilúvatar to the Númenóreans for breaking the Ban of the Valar. Ar-Pharazôn, the King of Númenor, had been coerced by Sauron to assault the Undying Lands which ultimately led to the island's destruction and death of most of its people in SA 3319.
Early in the Second Age, Morgoth's greatest servant, Sauron, had grown mighty in Middle-earth. Ruling through terror, force, and persuasion, Sauron dominated peoples of Middle-earth in the south and east. After the forging of the One Ring Sauron launched an invasion of Eriador and destroyed Eregion in the War of the Elves and Sauron. Númenor dispatched an immense military force to Middle-earth to aid the Elves of Lindon and after a period of heavy fighting, defeated Sauron and his hosts.
Sauron returned to Mordor to rebuild his strength and having tasted the power of conquest and victory in arms, the Númenóreans began establishing settlements along the coasts and oppressed the Men of Middle-earth. Many began to speak openly of their envy of the Eldar's immortality and in defiance of the Ban. It appeared to the Valar that the shadow of Morgoth had returned to plague their hearts. Emissaries of the Valar came to Númenor during the reign of Tar-Atanamir, seeking to remind them that death was not given as a punishment, but the people of Númenor did not heed their words and later became split into two factions; 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 loyal to the Valar. As time went on, Númenor continued to grow in wealth and might, yet its people's bliss was diminished.
Advisor to the King
After returning to the height of his power, Sauron began attacking Númenórean cities by the coasts and took on the titles 'Lord of the Earth' and 'King of Men'. Ar-Pharazôn the King received word of his rise in power, and angered by Sauron's pride the Númenóreans swiftly came once more to Middle-earth with a great military force to confront Sauron a final time in SA 3261. So mighty were they that Sauron's forces fled from them with barely a fight. Sauron realising he could not defeat the Númenóreans militarily allowed himself to be captured, and begged falsely for mercy. The Númenóreans brought him back as a prisoner.
Through Sauron's cunning and persuasion he rose from prisoner to adviser for the King, convincing many officials to worship Morgoth, they quickly became corrupted to his side. Under Sauron's influence the Númenóreans prospered but became more nefarious, the Men of Middle-earth were hunted and enslaved and brought back to Númenor be slain cruelly, sometimes sacrificed in the name of Morgoth. The White Tree, Nimloth, was cut down and its wood was used to light the first fire on the altar of a mighty Temple built in Armenelos. Recognising the corruption of the council Amandil, the Lord of Andúnië withdrew to Rómenna where most of the Faithful had been forced to relocate and were being openly persecuted.
The shadow of death soon loomed over Ar-Pharazôn and fearing death and old age the king was prompted by Sauron to make war upon the Valar, seize the Undying Lands and take immortality by force and so preparations began in SA 3310 of a great armada for the invasion. Amandil was aware of the king's machinations and consulted with his son Elendil, revealing his plan to sail west in secret, imitating their forefather Eärendil, and seek mercy from the Valar for what was to come, as Amandil perceived it would be the Doom of Númenor. Amandil then departed, along with three close servants he sailed east as if heading for Middle-earth, then turned the west and was never seen or heard of again. Following the advice of his father Elendil prepared for the ruin of Númenor, gathering more of the Faithful and prepared their ships that were anchored in the east coast, storing their heirlooms and goods.
Drowning of Númenor
During these days the weather in Númenor grew restless, and storms wrecked the ships of the Númenóreans. Then a great cloud "shaped as an eagle" loomed over the island and beneath its wings lightning struck the land. Few Númenóreans repented; most of them defied the warning, and Sauron stood on the temple roof unscathed by the lightning. By SA 3319, Ar-Pharazôn's armada was complete, known as the Great Armament. Ar-Pharazôn boarded his flagship Alcarondas ahead of his fleet and departed for Aman. Sauron remained behind. The fleet finally arrived on the shores of Aman and Ar-Pharazôn's host made their camp about Túna.
The Valar feared that this host could wreak 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 opened a massive chasm in the sea between Númenor and Aman that swallowed the Great Armament of Ar-Pharazôn. And the king himself, along with his host that had landed on Aman, was 'buried under falling hills', and would remain in the Caves of the Forgotten until the Dagor Dagorath. Númenor was drowned by a great wave and sank into the abyss, killing its inhabitants, including the body of Sauron, which robbed him his ability to assume fair forms ever after. Ilúvatar broke and changed the world, changing Arda's shape from flat to round and taking Aman and Tol Eressëa from the world forever, so that no mortal sailor could reach the True West again.
The Faithful managed to escape the catastrophe, led by Elendil and his sons Isildur and Anárion on nine ships. Among their possessions were the Palantíri, the Ring of Barahir, the silver rod of the Lords of Andúnië, Narsil and a scion of Nimloth which grew from a fruit Isildur saved before the tree was cut down. They sailed eastward and landed on Middle-earth, but due to the upheaval caused by the destruction of Númenor their ships were separated, such that Elendil landed in Lindon with four ships, while Isildur's three and Anárion's two were borne southward.
The followers of Elendil united the colonists of those lands among the Middle Men, and established two kingdoms which came to be known as the Realms in Exile: Gondor, in the south, and Arnor, in the north. These few Númenóreans endeavoured to preserve Númenor's culture in these realms. Adûnaic, the language of Númenor, was spoken in them; it later evolved into Westron or the Common Speech, borrowing some words from Elvish languages. The sadness and the shock from the loss of their island home always remained in the hearts of the Númenóreans and their descendants and was commemorated, in Gondor at least, by the Standing Silence.
Other Númenóreans survived that were of the King's Men, as they were already in Middle-earth before the Downfall and established other realms in exile to the south; of these the Haven of Umbar was the chief and continued to dominate the peoples of neighbouring Harad. They later became known as the Black Númenóreans and continued to serve Sauron, whom had also survived the Downfall, although greatly diminished and bereft of shape returned to Middle-earth with the One Ring to continue troubling its inhabitants.
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, leaving 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.
After its fall Númenor was called Akallabêth or Atalantë in Quenya, meaning "the Downfallen", Mar-nu-Falmar ("Land under the Waves").
Sauron's plan to destroy Númenor had been overwhelmingly successful, and though he too was caught by the flooding, he escaped, and at some point, returned to Middle-earth, believing himself to now be fully unopposed.