In 1358 DR, a warlock coven known as the Toril 13 executed a ritual that cursed most tiefling lineages—Those people of demons, devils, hags, and rakshasas, amongst Other people—Using the "blood of Asmodeus", https://crithitceramics.com/crafting-the-perfect-tiefling-warlock-a-guide-for-all-ages/