The downfall of Númenor in J.R.R. Tolkien's legendarium is called the Akallabêth. The name is Quenya for “The Downfallen” and also titles the account of Númenor’s destruction.
Númenor was an island kingdom founded in the Second Age for the Edain who had aided the Elves during the wars against Morgoth. Its people were granted long life, but they were forbidden to sail to the Undying Lands of Aman, because mortality remained part of the order of the world.
Over time, fear of death and resentment of the Ban grew among the Númenóreans. Sauron exploited these fears after he was taken to the island as a prisoner. Under his influence, King Ar-Pharazôn challenged the Valar and sailed west with a great fleet.
The result was the destruction of Númenor and the reshaping of the world. In Adûnaic, the event is also associated with the name Atalantë, meaning “Downfallen.”