The Age of Ash
Tieflings refer to the events of the world’s apocalypse as the “Night of Black Fire”...
...and some will brag that their ancestors were responsible for bringing it about. According to such dubious scholars, the Night of Black Fire is retribution upon the Dragonborn for the destruction of Bael Turath. They say that the Tieflings of antiquity somehow succeeded in opening a rift that linked the Nine Hells and the Abyss in a common ground on the natural world, and the resultant war brought about the apocalypse and the sealing of all of the planar gates.
Tieflings are rare enough that they almost never form singular racial groups any longer, instead lingering among Human and Shifter societies as either outcasts or leaders, respectively.