The fictional kingdom destroyed at the end of the Second Age is Númenor.
Númenor was an island realm granted to Men who had remained faithful to the Valar after the wars of the First Age. Its people were called Númenóreans, and their first king was Elros Tar-Minyatur, the half-elven brother of Elrond.
Over time, many Númenóreans became jealous of the immortality associated with the Elves and increasingly challenged the will of the Valar. The king Ar-Pharazôn eventually took counsel from Sauron, who encouraged Númenor’s rebellion and persuaded him to attack the Undying Lands.
The island was overwhelmed and destroyed by Eru Ilúvatar in the event known as the Downfall of Númenor. Survivors led by Elendil reached Middle-earth and founded Arnor and Gondor. Númenor is therefore a major ancestral link between the First Age legends and the political world of The Lord of the Rings.