The fictional island kingdom created by J.R.R. Tolkien that was destroyed before the Third Age is Númenor.
Númenor was an island realm in the Second Age, located in the Great Sea between Middle-earth and Aman. The Valar raised the island for the Edain, the Men who had aided the Elves during the wars of the First Age. The Númenóreans became renowned for their long lives, maritime skill, and powerful kingdom.
Over time, many Númenóreans grew jealous of the immortal Elves and feared death. Sauron exploited that fear after allowing himself to be taken to Númenor. He influenced King Ar-Pharazôn and encouraged an attack on Aman, even though the inhabitants of the island were forbidden to sail there.
The assault led to Númenor’s destruction in the event called the Downfall of Númenor. Elendil and his sons escaped to Middle-earth, where they established the realms of Arnor and Gondor. Númenor is therefore central to the history preceding The Lord of the Rings.