It tells the story of the Downfall of Númenor after the Númenóreans, the descendants of those Men who aided the Elves in their fight against Morgoth during the First Age, turned by degrees against the Valar, and were later corrupted by Sauron. The Akallabêth was written by Elendil near the end of the Second Age and was preserved in Gondor
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 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 good will 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.
The study of Númenor's history was suppressed in the realms of Exile because it was seen as a vain pursuit, "breeding only useless regret". Only one story from the former home of the Dúnedain remained generally known: the cautionary tale of the pride of Ar-Pharazôn and his "impious armada", which corresponds to the last half of the Akallabêth.
Akallabêth or Atalantë was the name for the island of Númenor after its Downfall. The Exiles became reluctant to speak of the land by any other name, or indeed much at all; but at times "they turned towards the West in the desire of their hearts" and remembered Akallabêth, their former home.
The Akallabêth originated with The Lost Road, a time-travel story that Tolkien wrote due to a conversation with C.S. Lewis about writing their own science fiction. Lewis produced Out of the Silent Planet however Tolkien abandoned The Lost Road, having only written two introductory chapters and two chapters of Númenor in the end, as he was more interested in writing his own version of the Atlantis legend. As Tolkien was writing The Lost Road he also produced another manuscript, an outline which was closely related to the last two chapters of The Lost Road about Númenor and its downfall. This was followed by another, more finished, manuscript titled The Fall of Númenor (chapter). These can be said to be the true germ of the Akallabêth itself, written in the mid-to-late thirties.
Tolkien began to develop the tale of Númenor's Downfall further when writing The Notion Club Papers in the 1940s, about a fictional discussion group called the Notion Club (a reference to the Inklings) where its members discuss Alwin Arundel Lowdham's dreams of Atlantis and Númenor. At this time there were major developments to the legend in The Drowning of Anadûnê. A large amount of the wording, especially in the later versions of the text, is retained in the Akallabêth. Christopher Tolkien notes in essays, which he named sketches written by his father that he was developing two traditions concerning Númenor's history, each with its own retelling of the Downfall: The Fall of Númenor, a more Elvish version, and a "Mannish" form, The Drowning of Anadûnê. These texts can be found in Volume IX of The History of Middle-earth, Sauron Defeated.
Tolkien's final revisions of the Akallabêth were published in the last volume of The History of Middle-earth, The Peoples of Middle-earth. It was from this version that Christopher got the document that he published in The Silmarillion. Since the latest version of the Akallabêth contained references to Ælfwine, Christopher removed all references to him, as he believed that his father abandoned the framing device of Ælfwine and Pengolodh because there was no suggestion of it in his father's latest writings of The Quenta Silmarillion before his death.
- Westernesse, an attempt by Helge Fauskanger to show how the Akallabêth could be adapted as a prequel to Peter Jackson's The Lord of the Rings and The Hobbit movies.
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "The Line of Elros: Kings of Númenor"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Akallabêth: The Downfall of Númenor"
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "A Description of the Island of Númenor"
- J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson), p. 111 entry Adûnaic: Akallabêth
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part Two: The Notion Club Papers Part Two: Night 67", p. 247
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Index of Names", entry Akallabêth
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part Three: The Drowning of Anadûnê: (vi) Lowdham's Report on the Adunaic Language: [Final section: Further material]", p. 439
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Peoples of Middle-earth, "V. The History of the Akallabêth", $83 note on the name Atalante
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Index of Names", entry Atalantë
- J.R.R. Tolkien; Humphrey Carpenter, Christopher Tolkien (eds.), The Letters of J.R.R. Tolkien, Note 102 to Letter 257, (dated 16 July 1964)
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Akallabêth: The Downfall of Númenor", third last paragraph
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Lost Road and Other Writings, "Part One. The Fall of Númenor and The Lost Road: I. The Early History of the Legend"
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part Two: The Notion Club Papers: Introduction"
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part Three: The Drowning of Anadûnê: (iii) The second text: [Untitled section: Comparison of texts]"
- J.R.R. Tolkien, Christopher Tolkien (ed.), Sauron Defeated, "Part Three: The Drowning of Anadûnê: (v) The theory of the work"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part One, "Foreword"