airwalkrr's Out of the Abyss
Tieflings are a fiendish subrace of the planetouched race, creatures whose origins lie in the outer planes. Some tieflings have fiendish blood and can draw a direct line of descent to a demon, devil, or daemon. Others were transformed by exposure to such creatures or by dwelling on an outer plane for too long.
Tieflings are exceptionally rare in the Flanaess, although they are more common than dragonborn. Most tieflings are found in service to Iuz, the worldly demigod of pain, or as servants of the Fiend-Seeing Throne of the Great Kingdom. The former are most commonly of demonic origin while the latter are usually of devilish origin. Even in the lands where they are more common, they are feared and reviled.
The appearance of tieflings varies according to their heritage, but it is always obvious to casual inspection. A tiefling might have cloven hooves instead of feet, ram-like horns atop the head, a barbed tail (not prehensile), red skin, or any combination of the above and more. A tiefling’s appearance often (but not always) bears some resemblance to the type of fiend from which the tiefling is descended or the fiend which caused the transformation.
House Rule: tieflings are almost always evil and never good in alignment unless some powerful magic caused an alignment shift, such as a cursed magic item of opposite alignment.