Simultaneously more and less than mortal, tieflings are the offspring of humans and fiends. With otherworldly blood and traits to match, tieflings are often shunned and despised out of reactionary fear. Most tieflings never know their fiendish sire, as the coupling that produced their curse occurred generations earlier. The taint is long-lasting and persistent, often manifesting at birth or sometimes later in life, as a powerful, though often unwanted, boon. Despite their fiendish appearance and netherworld origins, tieflings have a human's capacity of choosing their fate, and while many embrace their dark heritage and side with fiendish powers, others reject their darker predilections. Though the power of their blood calls nearly every tiefling to fury, destruction, and wrath, even the spawn of a succubus can become a saint and the grandchild of a pit fiend an unsuspecting hero.
Also known colloquially (and somewhat pejoratively) as 'hellspawn' or 'devilblooded', they are often distrusted and feared for this evil lineage. Even in infernal Cheliax where some tieflings serve as middlemen between the people and the denizens of Hell, they are second-class citizens at best.
Though tieflings have the blood of fiendish beings, their ancestry is at least one step removed from the original introduction of that blood; the child of a union of a mortal with a fiendish being is a half-fiend, not a tiefling. Tieflings might, but not necessarily, then be born from the union of the half-fiend with another mortal being.
Tieflings vary greatly in individual appearance from one another, depending not only on the ethnicity of the human parent but also the type of the fiendish parent. They can have horns, greatly varying skin colors, hooves for feet, tails, unusual eyes, and so on. These variances mean that two tieflings born from two different fiends might have vast differences in appearance.