Tieflings plancetouched, which means that they have the blood of outsiders running through their ways. More specifically, tieflings are humans with distant evil outsider ancestors. The children of, for example, demons, devils or rakshasa may produce tiefling offspring. Contrary to popular belief, they are not half-fiends.
Tieflings come in many shapes and sizes, but are always born of humans or other planetouched. They often look like their human parents but have a few features to set them apart from normal peope; it might be horns, fur, scales, black eyes or anything horrific. They’re treated as abominations and usually end up as loners, thieving, murdering or adventuring to get by. They’ve learnt to be suspicious of others since very few ever get accepted by society. Most tieflings are evil by nature.
Due to magical breeding experiments, there is a high tiefling population in the land of Thay. The tieflings there are often enslaved to thralldom or used as assassins and spies. Some manage to escape into the world. Mulhorand also produces many tieflings as a result of their religious practices.