The Night Purges
Almost immediately following The Great Marring that tore Arcadia apart, races almost uniformly hunted and killed anyone who they perceived as using “dangerous magics,” in hope of preventing another magical catastrophe.
Sorcerers, Warlocks, some mages, and virtually any group that communicated with otherworldly creatures beyond the Fey were pursued and killed by mobs of the paranoid.
The Tieflings received the worst of this hatred as the twisted descendants of people who dabbled in infernal powers. So great was the violence and injustice against them that The Tiefling Clans retreated into The Undercaverns to escape and form their own society.