It is a relatively short (about thirty pages long) summary of the Númenor narrative of J. R. R. Tolkien's legendarium. Númenor is the name of the island that is cataclysmically destroyed in Tolkien's variant of the Atlantis legend.
Akallabêth is the story of the destruction of the Kingdom of Númenor, written by Elendil. After the downfall of the Dark Lord Morgoth at the end of the First Age (which is described in the Quenta Silmarillion) the Edain, those Men who had aided the Elves in their war against the Dark Lord were given Númenor, a new small continent of their own, free from the evil and sadness of Middle-earth. It was located in the middle of the Great Ocean, between the western shores of Middle-earth, and the eastern shores of Aman, where the Valar lived.
As they entered Númenor, Men were forbidden to set sail towards Aman. For 2500 years Númenor grew in might, with Númenórean ships sailing the seas and establishing remote colonies in Middle-earth. During that time, the Elves of Middle-earth were engaged in a bitter fight with Morgoth's former servant Sauron, who had become the second Dark Lord. The Men of Númenor aided the Elves under Gil-galad yet remaining in Middle-earth. But as time went on, Men rebelled against the Valar and the Elves, over the course of 1500 years, desiring immortality. Tar-Palantir, the penultimate King, repented of the evil of his fathers, but it was too late.
The last king was his nephew Ar-Pharazôn, who claimed the throne by right of marrying Tar-Palantir's daughter Tar-Míriel against her will, despite the law not allowing cousin-marriages. Hearing that Sauron was striving for the domination of Men and threatening to destroy Númenor, Ar-Pharazôn came with a great host to Middle-earth. Sauron's forces feared Númenor's strength, and fled from the service of their master. Perceiving that he could not overthrow Númenor by strength of arms, Sauron humbled himself before the Númenórean King. Ar-Pharazôn was not convinced and had Sauron taken as a prisoner to Númenor. This however was Sauron's plan; assuming a form of great fairness and beauty, Sauron gained influence and became the king's adviser. Through deceit and manipulation, he corrupted the greater part of Númenor to the worship of Morgoth, which included raiding Middle-earth for human captives to offer as sacrifices and cutting down Nimloth, the White Tree, to build a Temple in its place. During this time, Númenor grew even more powerful thanks to Sauron's counsel.
Sauron convinced Ar-Pharazôn to assail Aman and wrest immortality from the Valar, saying that great kings take what rightfully belongs to them. Although Sauron wanted to destroy Ar-Pharazôn and the invading Númenórean fleets by provoking the Valar's wrath, he also wished to continue his absolute rule of that realm. However, when the Great Armament set foot on Aman, the Valar laid down their guardianship and called on Eru Ilúvatar, who broke and remade the world. Ilúvatar destroyed Ar-Pharazôn and his Númenórean host, burying them under falling hills until the end of time. Ilúvatar also sank Númenor into the Sea and removed Aman forever from the circles of the world. The world that had been flat was now spherical. Now Aman was only open to Elves, who could still find the Straight Road.
Nine ships carrying men of Númenórean royal blood, descendants of the Lords of Andúnië, of the House of Elros, were carried by the storm of the Downfall to the shores of Middle-earth. They were led by Elendil the Tall, and his two sons: Isildur and Anárion, bringing with them a seedling of the White Tree and the palantíri. These and the Númenóreans already living in Middle-earth carried the title of "The Faithful", signifying their continued devotion to the Valar and Eldar. They allied themselves with Gil-galad and marched in the War of the Last Alliance, in which Isildur cut the One Ring from Sauron's hand.
Elendil's followers established two Númenórean realms in exile: Arnor, the high kingdom, in the North, and Gondor in the south. Some of the King's Men, enemies of Elendil, established other realms in exile to the south; of these Umbar was the chief. The culture of Númenor became the dominant culture of Middle-earth. Arda was made spherical, and Aman was put beyond it, out of the reach of mortal men. Sauron's physical form was destroyed but his spirit escaped Númenor and returned to Middle-earth, taking up the One Ring.
- Tolkien in 1964 described the similarity of the word "Atalantë" to "Atlantis" as a happy coincidence: "It is a curious chance that the stem talat used in Q[uenya] for 'slipping, sliding, falling down', of which atalantie is a normal (in Q) noun-formation, should so much resemble Atlantis. Carpenter, Humphrey, ed. (1981), The Letters of J. R. R. Tolkien, Boston: Houghton Mifflin, No. 257, ISBN 0-395-31555-7 (1964)
- The word is recorded in The Notion Club Papers (written 1945), an Adûnaic fragment received by a protagonist of that story in a visionary dream, Êphalak îdôn Yôzâyan... Êphal êphalak îdôn hi-Akallabêth glossed "Far away / now [is] / [the] Land of Gift ... Far / far away / now [is] / She-that-hath-fallen".