Source: Advanced Player's Guide
Archive of Nethys: Tiefling
When the influence of a demon, devil, or other fiend infiltrates the bloodline of a mortal family, tieflings are the inevitable result. Tieflings carry the sinister mark of the fiendish planes upon their flesh, and their specific abilities and physical qualities vary according to their heritage. Hellspawn are those who descend from devils, grimspawn hail from daemonic influences, and pitborn bear the influence of demons, though these are only the three most common among a wide variety of tiefling lineages.
Generations might pass between a fiend’s direct influence and the time a tiefling child is born. For those born to ignorant or fearful parents, childhood is particularly hard, but even those whose families accept and nurture them face fear and prejudice from society as a whole. In some cases, this rejection encourages a tiefling to embrace the evil within their heritage, though others carve out a place and live a fulfilling life despite the challenges facing them.
Two tieflings, even siblings or twins, might not look similar at all, for the influence of fiendish lineage manifests in unique and unusual ways. These variations never make a tiefling’s appearance so strange as to obscure their humanoid ancestry, but horns, a forked tongue, vestigial wings, a tail, or a cloven hoof in place of a foot are all common and obvious signs of their heritage.
Tieflings tend to adopt the society and culture they were born into. Since most societies consider them to be untrustworthy or even monstrous, tieflings typically gravitate toward those segments of society willing to overlook them entirely, or where their reputation can assist rather than hinder them. With determination and persistence, however, tieflings can earn respect and prestige despite their heritage, finding people who accept them as they are. In regions where worship of fiends is widespread, tieflings can live more openly and achieve great power and respect, although exceptions exist—in the Asmodean nation of Cheliax, for example, tieflings are seen as shameful proof of a failure to maintain control in the face of fiendish influence, and as such are cast out.