Once tieflings were just ordinary humans. Their ancestors were the noble class of the Empire of Bael Turath who sought to find ways to increase their hold on power. To that end they made deals with the powers that ruled the Nine Hells. These pacts were binding not only on those who made them, but on all of their blood both living and unborn. Their desires were granted and the tiefling race was born.
Tiefling have a reputation for being sneaky and underhanded. Many people grow suspicious of more than one or two tiefling in the same place at one time. For that reason, many tieflings become adventurers rather than settling down in one place. Although the bloodlines of Bael Turoth are the most common and the most recognizable tieflings, other evil outsiders have also used their foul seed to corrupt humanoid bloodlines. While rare, there are tieflings who have the blood of demons or worse running through their veins.