In Tolkien’s mythology, the great island kingdom destroyed in the Second Age is Númenor.
Númenor was an island realm in the Great Sea, created for the Men who had aided the Elves during the wars against Morgoth. Its people became powerful mariners and founded settlements and kingdoms in Middle-earth, including Gondor and Arnor through Elendil and his sons.
Over time, many Númenóreans became obsessed with avoiding death. Sauron influenced King Ar-Pharazôn, who attacked Valinor in defiance of the Ban of the Valar. As a result, Eru Ilúvatar changed the shape of the world and caused Númenor to sink beneath the sea.
Númenor is also called Westernesse or the Downfall of Númenor is called the Akallabêth. It is distinct from Beleriand, a region destroyed much earlier at the end of the First Age.