The island kingdom founded by Elros in J.R.R. Tolkien’s legendarium is Númenor.
The Valar raise Númenor from the sea as a reward for the Men who fought against Morgoth in the First Age. Elros, the half-elven brother of Elrond, chooses the fate of Men and becomes the first king of the island. His descendants rule Númenor for many generations and later establish realms in Middle-earth, including Gondor and Arnor.
Númenor’s people receive long life and great knowledge, but their fear of death gradually intensifies. Under King Ar-Pharazôn, Númenor challenges the authority of the Valar and invades Aman. The island is then destroyed and drowned, an event known as the Downfall of Númenor. Númenor is therefore distinct from Valinor, the land of the Valar, and from Beleriand, the largely destroyed region central to many First Age stories.