The Downfall of Númenor was the utter destruction of the island of Númenor, by the power of Ilúvatar, as punishment for Ar-Pharazôn's attempted assault on the Undying Lands.

When the Edain were gifted the land of Númenor for their alliance with the Eldar during the Battles of Beleriand, they were held to the Ban of the Valar. They were not to turn their ships so far into the west that they could no longer see their own shores.

Most of the Kings of Númenor obeyed this law, but soon they began to wonder why they could not partake of the beauty of the Blessed Realm. Some of them turned their hearts against the Valar and the Eldar and the shadow began to fall upon them.

The Númenóreans prospered, but always they yearned to be deathless, and as the years went on they began to greatly desire seeing Aman. Sauron was able to use the fact that Men died and feared death to his advantage, at last convincing the last of the Kings of Númenor that he should sail to the Undying Lands and take them for his own.

This Ar-Pharazôn did and he, along with his fleet of the King's Men, caused the Downfall of Númenor. Only Elendil and his companions of the Faithful escaped the disaster.

The land of Númenor was swallowed into the sea and was ever after called by many other names: Mar-nu-Falmar, Akallabêth, the Downfallen, and Atalantë.