Which Tolkien book contains the account of Númenor’s downfall called the Akallabêth?

The Silmarillion contains the account of Númenor’s downfall called the Akallabêth.

Akallabêth is one of the major sections of The Silmarillion. Its title is a Quenya word meaning “The Downfallen,” and the narrative describes the destruction of Númenor, the island kingdom of Men descended from the Edain who aided the Elves during the First Age.

The story centers on Ar-Pharazôn, the last king of Númenor, and his conflict with Sauron. Manipulated by Sauron, the king’s followers attack Valinor in defiance of the ban of the Valar. Eru Ilúvatar then changes the shape of the world and destroys Númenor, while Elendil and his sons escape to Middle-earth.

The account is sometimes associated only with The Lord of the Rings because Númenórean history affects Gondor and Arnor. However, the detailed narrative appears in The Silmarillion, published in 1977 after Tolkien’s death.