Half-Fiends and Tieflings

Tieflings come from somewhere. While the story of a nefarious deal made by great-great-great-great grandma causing a tiefling generations later, or dear old pa accidentally doing a favor for a devil and getting a child in exchange isn’t uncommon, many questions arise from tieflings who simply shrug their shoulders on their origins. One can easily think of the other answer, yet rarely does coupling between a humanoid and a fiend result in a child at all, and what’s born nine months later is not quite a tiefling.
Half-fiends were far more common before the invention of society led to the novelty of witch hunts, and then witch hunts led to the novelty of evil always coming from somewhere else. Possessing more traits of their fiendish ancestors, half-fiends are immediately noticeable, unless they have access to magic to hide their appearance. For instance, someone with a babau parent will have limbs that are far too long, and completely smooth, hairless skin. With this trade off however, comes a deeper connection to the profane. A half-fiend has connections to darker magic, without the effects that would typically dirty or ensnare their soul.
If a half-fiend has a child with another humanoid, that child will either seemingly be a normal member of their kind, or be a tiefling themselves. Even if they share the ancestry of their humanoid parent, they will still retain the possibility of having tiefling children just like those who have had their bloodlines touched in other ways.