History of the Drow, The Descent:

We know very little of the Ilythiiri, or "Elves of the South," before this crucial event. Even then they were known as "Dark Elves," for the hue of their skins. They dwelt in the jungles and hot forests of the South. A proud, warlike, culturally advanced (some sages of other elven peoples say "decadent") folk, the Ilythiiri attacked all neighbors, including other elven tribes. Their cruel raids and depredations, ordered by warlike nobility and the clergy of their two cruel deities, Ghaunadaur and Lolth, forced elves, humans, dwarves, and others to ally against them.

Defeated in a series of titanic magical battles, the dark elves fled into underground warrens they had earlier discovered. This event, known as "the Descent," marked the end of the drow as a surface-dwelling race.