The island kingdom destroyed near the end of Tolkien’s Second Age was Númenor. Its destruction occurred in the year 3319 of the Second Age, after its king Ar-Pharazôn challenged the divine powers.
Númenor was founded as a reward for the Edain, the Men who had aided the Elves during the wars against Morgoth in the First Age. Its people became renowned for long life, maritime skill, learning, and powerful ships. Over time, fear of death and resentment toward the immortal Elves grew among many Númenóreans.
Sauron was taken to Númenor as a prisoner but eventually became a powerful adviser to Ar-Pharazôn. He encouraged the king to attack the Undying Lands in an attempt to seize immortality. The island was then overwhelmed and changed in the reshaping of the world.
Elendil and his sons survived the catastrophe and founded the realms of Arnor and Gondor in Middle-earth. Númenor is sometimes called Westernesse, but that is an English name for the same island rather than a separate kingdom.