The Realm of Eärwa
How Tieflings came to be
Tieflings came from all of the Prisoners of War taken during and at the end of Lightfall, the sick corruptions come from the grove within the Empire of 1,000 Leaves. Failures include the Black Goblins, Black Cyclopes, and Black Trolls.
With that in mind, Tieflings started appearing 180 years ago, and have since had family units (while Limited) forming.