The fictional island kingdom J.R.R. Tolkien placed in the Second Age of Middle-earth is Númenor.
Númenor is an island realm in the Great Sea, west of Middle-earth. The Valar raise it for the Edain, the Men who aided the Elves during the wars against Morgoth. Its first king is Elros, the half-elven brother of Elrond, and its people receive unusually long lifespans as a gift after the First Age.
Númenórean history eventually turns tragic. The island’s rulers grow jealous of the immortal Elves and fear death, while Sauron influences the late king Ar-Pharazôn. After the king attempts to invade Valinor, Eru Ilúvatar changes the shape of the world and Númenor is destroyed. Survivors found the realms of Arnor and Gondor. Númenor is therefore both an ancient civilization and the ancestral source of Aragorn’s royal line.