The island kingdom destroyed by a great wave in Tolkien’s legendarium is Númenor.
Númenor is granted to the Men who aided the Elves during the First Age. Its people become powerful mariners, but their fear of death grows into resentment toward the immortal Valar and the Elves. Under King Ar-Pharazôn, Númenor’s rulers challenge the order of the world.
The island is destroyed when Ar-Pharazôn’s armada attempts to invade Aman. Tolkien presents this catastrophe as a mythic punishment, after which the world is changed from flat to round. Survivors establish realms in Middle-earth, including Arnor and Gondor.