Drow

Drow, or dark elves, are the descendants of the elves who refused to abandon Golarion when it was discovered that the Starstone would hit the world. Unlike their cousins, they did not escape through a dimensional portal, but rather hid themselves in the Darklands below. Something changed these elves during their stay, and when the other elves returned, they found their brethren bitter, twisted, and evil enemies.

Origin of the drow

During the Age of Legend, the ancestors of today's drow race were simply elves living in the northwest corner of Avistan, in a small forest kingdom around the city of Celwynvian. An elven secret society known as the Winter Council had discovered that the antediluvian aboleth race was behind the rise of the human civilizations of Azlant and Thassilon. They also learned that the aboleth had grown tired of humanity's arrogance and planned to destroy them by calling down a meteor from the heavens.
With time to avoid this cataclysmic event, the elves called out to their brethren to flee Golarion for their ancient refuge of Sovyrian on the planet Castrovel, via the powerful aiudara gate known as the Sovyrian Stone. Most did so, but a few elves refused to abandon Golarion. The exact reasons why they chose to stay behind is still a matter of debate among the elves. Some believe it was out of loyalty to this world, while others suggest that the drow were a political faction who were against the passive elven response to humanity's encroachment on their land.
Whatever the true reason, a group of elves fled into the Caves of the Craven below the Calphiak Mountains, believing that they could escape the surface world's destruction by simply delving deep enough into the Darklands. When Earthfall struck, the Darklands were shaken by tremendous tremors, strong enough to briefly wake Rovagug, the god of destruction who had been imprisoned far below during the Age of Creation. As his wakened mind reached out, it touched the cave where the elves had sequestered themselves, and mixed with the Darklands' strange radiations and the elves' own feelings of betrayal and anger. It transformed them, turning their hair white and skin dark, and replaced the race's natural tendencies towards independence and good with Rovagug's evil; these elves became the first generation of drow.