The great island kingdom destroyed by the sea in J.R.R. Tolkien’s mythology is Númenor.
Númenor was an island realm granted to the Men who had aided the Elves during the First Age. Its people, the Dúnedain, enjoyed long lives and became a powerful maritime civilization between Middle-earth and the Undying Lands.
Over time, many Númenóreans grew jealous of the immortality associated with the Elves and the Valar. Under King Ar-Pharazôn, the kingdom challenged the divine order and attempted to reach Aman by force. Tolkien’s account of its downfall is known as the Akallabêth.
Númenor was destroyed in the Second Age, and its survivors founded the realms of Arnor and Gondor in Middle-earth. It is therefore an ancestor of the kingdoms ruled by Aragorn in The Lord of the Rings.