Little Devil Kids
Metaphysical / Paranormal event
Circa 1,000 AC
The first recorded instance of tieflings was a skirmish that occurred when an "army" of "devil-people" suddenly "appeared" in the space that the city of Chamenos once occupied.
This so-called army was quickly dispersed when local lords sent soldiers to deal with the "extraplanar" threat, but tieflings have been kicking around ever since, albeit at a healthy distance from those who distrust them simply because they have horns (and also the whole infernal heritage thing).