What is the name of the cataclysm that destroys Númenor in J.R.R. Tolkien’s legendarium?

The destruction of Númenor in J.R.R. Tolkien’s legendarium is called the Downfall of Númenor.

In Tolkien’s account, Númenor is a powerful island kingdom granted to Men after the defeat of Morgoth. Its people become increasingly resentful of their mortality and envy the immortality of the Elves. Sauron exploits this fear and helps persuade King Ar-Pharazôn to challenge the divine order.

Ar-Pharazôn leads a fleet toward Aman, the Undying Lands, seeking immortality. In response, Eru Ilúvatar changes the shape of the world, destroys Númenor, and removes Aman from the ordinary world. The event is described in the chapter “Akallabêth,” a name meaning “The Downfall” in Adûnaic.

Survivors led by Elendil and his sons escape and establish kingdoms in Middle-earth. The Downfall is therefore both a catastrophe and a turning point in the history of Gondor and Arnor.