Magical Collapse

The Magical Collapse was a catastrophic event that brought on the end of the First Age, and the destruction of the ancient civilization of the elves. The truth of what caused it is unknown, however, it is largely speculated it was caused by the wars between the different elves at the end of the First Age.

Aftermath
With the Magical Collapse, the Second Era came to an end. Many of the greatest elven cities collapsed entirely, as the magic sustaining them was no longer possible. Ironically, it was the weakest and most backwards of elven lands that managed to survive.

The ancestors of the high elves did their best to keep their ancient civilizations intact, with varying success, while many of the ancestors of the drow fled underground to avoid the aftermath. The ancestors of the wood elves saw it as a sign from the gods that their way of life was the correct one, and spread across the wilderness.

It is theorized that the Magical Collapse also played a hand in the physical alteration of the elves, causing them to split into three distinct sub-groups. The Magical Collapse also brought the substance Godsblood into the Surface World, which became the cornerstone of the Dwarven Empire of the Second Age.

Trivia
The Magical Collapse is the equivalent for the Spellplague for the Undergod's Realm setting.