The island kingdom that falls in J.R.R. Tolkien’s legendarium is Númenor.
Númenor is an island realm in the Second Age of Tolkien’s history. The Valar raise it from the sea for the Edain, the Men who aided the Elves during the wars of the First Age. Its people become powerful sailors and establish colonies and strongholds across Middle-earth.
Over time, Númenórean rulers grow increasingly concerned with death and seek a life that is forbidden to Men. Under King Ar-Pharazôn, the kingdom challenges the divine order by sailing against Aman. The resulting destruction sinks Númenor beneath the sea, an event known as the Downfall.
Númenor is sometimes confused with Valinor, the land of the Valar. Valinor lies in Aman; Númenor was a separate island kingdom created for Men.