One of your distant ancestors was an evil outsider. The characteristics you display might depend in part on the kind of fiendish heritage you have. Many of your bloodline are dark in coloration, although great variation can exist depending on your specific ancestry.
Evil outsiders such as demons and devils produce a surprisingly large number of offspring on the Material Plane. Their descendants are frequently (but not always) evil. Demon-blooded characters and others descended from beings native to the chaotic evil realms are usually chaotic themselves, and at their worst they can be prone to greedy or lustful behavior
Those with devil blood, on the other hand, tend to be lawful. They have no compunctions about bending the rules as far as possible, but they rarely break them. Like their fiendish ancestors, devil-spawn are known for keeping their word.
Characters with fiendish blood often seem driven— even haunted—by the potential for evil within them, especially if they try to hold to a neutral or good path. They frequently adventure out of a restlessness caused by their own internal struggles, although sometimes they do so simply because they enjoy killing and adventuring provides a perfect outlet for that tendency. Such characters usually prefer spells of darkness and deception, even if they are good-aligned.
Tieflings appear human at a glance, although they all possess at least one unnatural feature that reveals their infernal heritage. At the least, tieflings have a disturbing demeanor or carry a whiff of brimstone about them, but most also possess small horns, red eyes, or needle-sharp teeth. In extreme cases, a tiefling may have a barbed tail or cloven feet. No two tieflings have the same appearance.
Shunned and maligned by the world at large, tieflings maintain a low profile and make their livings as spies, thieves, assassins, or smugglers. Few find the “legitimate” world very appealing, and instead they blossom in the seedier side of society. Tieflings are diametrically opposed to aasimars, and the two have found themselves innately at odds for untold generations. Some tieflings serve as mere soldiers for the dark powers, while others devise their own selfish goals, seeing cooperation with infernal beings formutual benefit.
Tieflings have a well-deserved reputation for being conniving, sneaky, and underhanded. They are universally self-serving and only look out for number one. Some take their evil nature to an extreme and become champions of woe, despair, and violence. Most tieflings, however, prefer to skulk behind thescenes, using deceit and surprise as their primary weapons. In rare instances, a tiefling fights her natural impulses toward evil and strives to do good (or at least not to cause harm). These repentant beings face persecution and mistrust from others of their kind.
Creating a Tieflinga “Tiefling” is an inherited template that can be added to most standard humanoids (referred to hereafter as the “base creature”). The creature’s type changes to “Fel.” It uses all the base creature’s statistics and special abilities except as noted here.