Akallabêth is the Quenya name for the destruction of Númenor in J.R.R. Tolkien’s legendarium.
The term is also the title of a section in The Silmarillion that recounts Númenor’s downfall. The island kingdom was founded for Men who had aided the Valar during the War of Wrath, but its rulers later became increasingly fearful of death and jealous of the immortal Elves.
Under King Ar-Pharazôn, Númenórean forces challenged Sauron and brought him to the island as a prisoner. Sauron corrupted the king, encouraged the worship of Morgoth, and persuaded him to attack the Undying Lands. In response, Eru Ilúvatar changed the shape of the world and submerged Númenor.
“Akallabêth” is distinct from the War of Wrath, which ended the First Age. Númenor’s destruction occurred much later, near the end of the Second Age.