Ereburst

The Ereburst was the second major magical burst into the world. When the Terrasque Slayers killed Shadir - God of Hatred at The Well, they also forced Asch Figthorn to cast his own magical spark out into the surrounding area, destabilizing the beam of pure Erebus swirling out of the origin point of the world.
The subsequent explosion separated the Erebus into its two components, Aether and Deridah. The elements dissipated across the world, settling into people at random. Ones touched by significant amounts of Deridah were forever changed into Tieflings, and those touched by significant amounts of Aether were turned into Aasimar.