Military action
The hierarchy of high elven society was too rigid for the lower classes to bear. The goddess Loth tried to convince her father Corillion to change things to no avail. So she tricked her sister Selene to kill their father starting a civil war. The drows lost and were exiled to the undrdark.
The hierarchy of the high elven kingdom was structured in a way that the low-born elves had almost no rights and the nobles were treated as gods, in particular, the female elves were only seen as workers and mothers, not intelligent enough for politics or magic the two most important roles in the high elven society. The goddess Loth, goddess of tactics, rebellion, spiders, and one of the goddess of femininity, began to feel the suffering of her worshippers, who started to indulge in depraved and sadistic tendencies to feel the power that they lacked and relieve stress. Loth begged her father Corellion to do something to change the structure and give more power to the lower classes, but Corellian was too preoccupied with the war with the orcs to do anything about that. Loth then instructed her sister Sehanine, the goddess of the moon, hunting, compassion, and at the time archery, to post on a mountain from which to see the next battle but far away enough so that Corillion couldn't see her, for it was forbidden for female elves to enter a battlefield. Once posted on the mountain she would shoot Grumpsh the warrior god of the orcs with her mighty hunting bow, and in doing so help the elves win the war, so that Corillion could then solve Loth's problem. What Sehanine didn't know was that Loth was planning to kill their father to destroy the hierarchy of the high elves, for her father would not change anything, he liked the society the high elves had built in his honor. So when the moment came, Loth cast an illusion making it seem that Corillion was Grumpsh and Grumpsh was Corillion, Sehanine shot straight through her father's eye and shattered him into the minor high elven pantheon. The elves loyal to Loth calling themself the drow (that meant lowborn at the time but now is used to define the dark elves) then rebelled and rioted in each city burning the temples to Corillion and Sehanine and erecting monuments to Loth. Then started the high elven civil war, the drows were defeated and exiled to the Underdark cursed to never see the sun. Loth was cursed too, to have the lower body of a spider, (Fling the world in elvish for spider is the same word used for traitor), and Sehanine never used a bow again, ceasing the title of the elven god of archery to Kurnos one of the new gods who came from the fragmentation of Corillion.