The island kingdom destroyed in the Downfall of J.R.R. Tolkien’s legendarium was Númenor.
Númenor was created as a reward for the Edain after the War of Wrath at the end of the First Age. Its people became renowned for long life, seafaring, and learning. Over generations, however, fear of death and resentment toward the immortal Elves and the Valar grew among many Númenóreans.
Sauron eventually corrupted the king, Ar-Pharazôn, and persuaded him to attack the Undying Lands. The Valar appealed to Eru Ilúvatar, who changed the shape of the world and caused Númenor to sink beneath the sea. This catastrophe is called the Downfall of Númenor or Akallabêth.
Survivors led by Elendil escaped and founded the realms of Arnor and Gondor in Middle-earth. Númenor is therefore a major predecessor of Gondor, not an Elven land such as Valinor or Beleriand.