Akallabêth means Downfallen, and like the Elvish word Atalantë of the same meaning is the name given afterwards to the great human land in the western ocean that was drowned beneath the waves for its arrogance. It is the invention of J.R.R. Tolkien, and the name Akallabêth is also the title of one of his tales, published appended to The Silmarillion in 1977. From childhood he was haunted by the vision of a land consumed beneath rising waves. His Atlantis-like legend of Númenor is how he incorporated it into his mythos.

The Akallabêth (an Adûnaic word, i.e. in the language of the Men of the West, its inhabitants) tells of the rise, of the greatness and the pride, and of the downfall, of the island raised up out of the Ocean for the tribes of Men who aided the Eldar (the Elves) in their war against Morgoth the Great Enemy. These were known as the Dúnedain, Men of the West; and the land created for them was called Númenórë (in their own language Anadûnê and in ours Westernesse), and Elenna (the Starward), and Andor (the Gift). But in after-times it was called the Downfallen.

Elros Tar-Minyatur was the first king, brother of that immortal Elrond the half-elven who chose immortality and comes into later tales. King Elros chose mortality but reigned over four hundred years, and the span of his descendants was immense. But as time went on they regretted their mortality (their "gift"), and the Ban of the Valar that prevented them sailing westward to the Undying Lands.

In the days of King Tar-Atanamir there came messengers from the undying West telling them not to seek also the gift of the Elves, that of dwelling eternally in sight of the blessed lands. But Tar-Atanamir refused to lay down his life in its vigour, as the custom was, and ended his kingship feeble and senseless, the first time.

Around this time they neglected the temple of Meneltarma in the height of the island, and Nimloth the scion of the White Tree. Instead of helping the uncivilized people of Middle-earth they set themselves up as masters, lords, and tribute-takers. The nineteenth king forsook the language of the Elves and ascended the throne in the Adûnaic tongue, Ar-Adûnakhor, the Lord of the West, which was blasphemous. Those who remained friendly with the Elves of the West, and with Gil-galad the Elvish king in Middle-earth, were known as the Faithful, a persecuted minority.

The Akallabêth tells how the last king of Númenor, Ar-Pharazôn the Golden, built a mighty fleet. First he sailed to Middle-earth and subdued Sauron, greatest surviving servant of the Evil One, and took him prisoner back to Númenor. Sauron became his adviser and ruled his realm, which turned to darkness: human sacrifice filled the temple of Meneltarma and the White Tree was burned.

Finally Ar-Pharazôn defied the Ban of the Valar and sailed westward, beyond Avallónë the home of the Elves, to Aman the blessed. The Valar laid down their guardianship of the world, and called on Eru, the One, who changed the shape of the world.

Then suddenly fire burst from the Meneltarma, and there came a mighty wind and a tumult of the earth, and the sky reeled, and the hills slid, and Númenor went down into the sea, with all its children and its wives and its maidens and its ladies proud; and all its gardens and its halls and its towers, its tombs and its riches, and its jewels and its webs and its things painted and carven, and its laughter and its mirth and its music, its wisdom and its lore: they vanished for ever.