The fictional island kingdom destroyed in J.R.R. Tolkien’s Akallabêth is Númenor.
Númenor was an island realm in the Second Age of Middle-earth, created as a gift for the Men who had aided the Elves during the wars against Morgoth. Its people became renowned for long life, seamanship, learning, and power. Over time, however, fear of death and resentment toward the immortal Elves intensified among many Númenóreans.
The kingdom’s downfall is described in “Akallabêth,” one of the sections of The Silmarillion. Under the influence of Sauron, King Ar-Pharazôn challenged the authority of the Valar and sailed west to attack the Undying Lands. The island was then overwhelmed and destroyed, while the world itself was changed.
Survivors led by Elendil escaped and founded the realms of Arnor and Gondor in Middle-earth. Númenor is therefore closely connected to Aragorn’s ancestry and to the later history of the Dúnedain.