Six Points of Light
The destruction of the world as it was did more damage than to the Material Plane alone. Bereft of their leaders, the powers that once enacted the will of the gods fractured. Among the more lawful outsiders, some order was restored, and given the strength and ideals of the creatures at the top, little changed.
But the changes were more than political. The boundaries between the planes themselves cracked, causing power to leak into the Material Realm.
Whether because of the influence of fiends, dangerous radiation from the Pyramids, the subtle powers leaking through planar rifts, the awakening of latent bloodlines, or some other cause, children began to be born with a fiendish look and powers born of such darkness.
Out of self-defense, the tieflings banded together early on, and this tendency shows still among the Old Families, noble houses that birth the majority of the world’s tieflings.
With their otherworldly origins, tieflings are well-known to inherit or bargain for strange powers.