Tiefling

Tieflings are fiend-touched in some manner. The stereotype is that a woman having a tiefling child means she had relations with a fiend, but the reality is that there are many ways fiends can have influence to cause tieflings to be born.

Aside from physical relations with mortals, another common one is a mortal making a pact with a fiend; as part of the payment they hide their power in the bloodline and wait for the ideal child to be conceived and use their powers to touch it before it's born, causing it to be born a tiefling.

There may yet be more ways for someone to be born a tiefling as well, but these are the commonly understood ones.