Lloth's betrayal of Corellon leads to the creation of the Drow.
After the events of the First Orc-Elf War, one of Corellon's assistants, Lolth, grew envious of the Alflord's power and sought to draw the Elves of Azthir-Terra into her web. Some followed, but many did not. In a bid to reconcile these two factions of Elves, Corellon and Lolth made their cases before the Elves. During the debate, Lolth sought to strike a mortal blow against Corellon. His followers blocked the worst of the attack, but Lolth's followers stood by and did nothing. After her failure, Lolth retreated to the Abyss, where she became a Demon Lord. But her betrayal split the Elves into the high and wood elves of the light and the surface, while Lolth's followers were twisted in her own image as the dark Drow who retreated into the Underdark.