The island kingdom founded by Men in J.R.R. Tolkien’s Second Age is Númenor.
Númenor was raised from the sea by the Valar as a reward for the Men who had fought alongside the Elves against Morgoth in the First Age. Its people, the Númenóreans, received an unusually long lifespan and became powerful mariners, builders, and rulers.
The kingdom’s history eventually turns tragic. Many Númenóreans grew resentful of the Elves’ immortality and feared death. Under Sauron’s influence, King Ar-Pharazôn attacked Valinor, and the island was destroyed in a cataclysm. Elendil and his sons escaped and later founded the realms of Gondor and Arnor in Middle-earth.
Númenor is sometimes confused with Gondor because both are linked to Elendil’s descendants. Gondor and Arnor were successor kingdoms on Middle-earth, whereas Númenor was the earlier island realm in the western sea.