Tieflings

Tieflings are one of the most terrible schemes of the Fiends. As a curse, condition of a pact, or simply by a mortal letting themselves be fooled by the velveted words of a Fiend, children are marked by the influence of the Lower Planes. This gives them power linked to their ancestry, like skin that resists heat or cold depending on the Fiend affecting them.
However, beyond their appearance being modified to appear fiendish, there is nothing special about them. That is their cross. Some paranoid mortals treat Tieflings like a bad omen, like an unlucky black cat that needs to be driven away.
With this, Fiends have only one goal: create souls that resent their condition and let themselves dive into the evil that is associated with them; or make normally good people be taken by the wave of hatred for something they do not perfectly understand.
In any case, there is potential to generate souls considered Evil by the gods, perfectly suited for fighting the eternal wars of Fiends in the Lower Planes.