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