The fictional island kingdom destroyed before Númenórean survivors reached Middle-earth is Númenor. In Tolkien’s mythology, Númenor was a powerful island realm granted to the Men who had supported the Valar during the First Age.
The Númenóreans were long-lived and became skilled sailors, but their leaders increasingly desired immortality. Sauron manipulated King Ar-Pharazôn, encouraging him to challenge the divine order and sail against the Undying Lands. The resulting disaster destroyed Númenor in the Second Age.
Elendil and his sons, Isildur and Anárion, escaped the Downfall and founded realms in Middle-earth. Their descendants became central to the histories of Arnor and Gondor, linking Númenor directly to Aragorn’s ancestry.
Valinor is the realm of the Valar, while Eregion and Gondolin are separate places in Middle-earth’s history. Númenor’s destruction is also called the Downfall of Númenor.