The island kingdom destroyed before the end of the Second Age in J.R.R. Tolkien’s legendarium is Númenor.
Númenor is founded in the Second Age for the Edain, the Men who aided the Elves during the wars against Morgoth. The Númenóreans receive exceptionally long lives and become renowned as sailors, builders, and rulers. Over time, however, fear of death and resentment toward the immortal Elves grow among their kings.
Sauron is captured and taken to Númenor by King Ar-Pharazôn, but he corrupts the king and encourages the worship of darkness. Ar-Pharazôn eventually sails against Valinor, attempting to seize immortality. In response, Eru Ilúvatar changes the world, destroys Númenor, and removes the direct route from Middle-earth to the Undying Lands.
Elendil and his sons survive the catastrophe and establish the realms of Arnor and Gondor. Valinor is the land of the Valar, while Beleriand is an earlier region destroyed at the end of the First Age.