Calamity

The Calamity was one of the notable battles during the ongoing Blood War between celestials, the abyss, and the hells. It marked the end of the previous age, as the calendar was reset, the Council of the Gods changed, and an entire civilization was wiped from existence. 
Asmodeus, his sights set on the material plane for conquest, came up with a plan to trick the agents of the abyss to help him in that conquest. During the course of the Calamity, Asmodeus was supposedly killed and quartered, and pieces of him were spread across the material plane to be later put together for his "rebirth." As part of this, agents of [Hell/Abyss?] attacked the race known as the sky elves, their technology unique and desirable to them. The Council did not take any measures to intervene, having decided as a group that the sky elves were creating far beyond what was safe for mortals, and that their punishment would be extermination.