The island kingdom founded by the Edain after the First Age in J.R.R. Tolkien’s mythology is Númenor.
Númenor is a great island realm in the Second Age, created as a reward for the Edain who aided the Valar and the Elves during the wars of the First Age. Its people, the Númenóreans, receive long life and exceptional skill, though they remain mortal. The island lies in the Great Sea between Middle-earth and Aman.
The kingdom grows powerful through seafaring and establishes colonies and strongholds along the coasts of Middle-earth. Over time, many Númenóreans become resentful of death and suspicious of the immortal Elves. This conflict intensifies under Sauron’s influence and leads to Númenor’s destruction in the Second Age.
Númenor is sometimes confused with Gondor or Arnor. Those are kingdoms founded later in Middle-earth by Elendil’s sons, Isildur and Anárion, after the Downfall of Númenor. Beleriand, by contrast, is a western region associated mainly with the First Age.