What cataclysmic event ends the First Age?

The cataclysmic event that ends the First Age is the War of Wrath. It was the final and largest conflict between the forces of the Valar and the armies of Morgoth, the first Dark Lord.

The war began when Eärendil reached Valinor and pleaded with the Valar to help the peoples of Middle-earth. The Valar responded by sending a great host of Valinor, including many Elves, against Morgoth. Their intervention marked the first time the powers of the West openly returned to Middle-earth in force.

The fighting devastated Beleriand, the western region where most of the First Age stories take place. Morgoth was defeated, captured, and cast into the Void. His lieutenant Sauron survived the conflict, but he was not yet the dominant Dark Lord of the later ages.

The destruction reshaped the world. Beleriand was largely submerged beneath the sea, and many surviving Elves travelled west to Valinor. The War of Wrath therefore closes the First Age both politically—with Morgoth’s defeat—and geographically, through the ruin of Beleriand.