The following is adapted from information provided in the AP:
Despised as half-breeds at best, hunted down as irredeemable fiends at worst, tieflings bear the taint of the Lower Planes and an unmistakable ancestry of evil. The scions of polluted bloodlines, tieflings are as much a curse as a race, not for any unified evil that their blood compels them to, but for the corrupted forms that forever set them apart from the rest of mortal kind, driving them towards acts of desperation, viciousness, and despair. The spawn of fiends and humans, tieflings are monsters not by nature or by any possession of their heritiage, but by the hatred and prejudice of a fearful world. Theirs is a corrupt existence, but not one without choice, and the light that rises from darkness shines even stronger than those that glow during the day.
Tieflings might hail from any region and innumerable circumstances, and as such have little in the way of a unified culture. Although their physical characteristics and similarly tainted ancestry define them as a race, they have no creation myth, no shared origin, no larger identity, and no true sense of unity or companionship. Yet what they do share is the scorn of others and lives that both unify and separate them as a strange, unnatural race, seemingly never intended for existence.
As descendants of mortals and evil outsiders, tieflings have no unique racial origin. They come from all walks of life, all geographical areas, and all shades of evil.
The commonly imagined origin of these half-breeds involves a fundamentally evil creature fraternizing with a depraved human. That scenario raises an important point, though: why are tieflings ever born?
This complex question has many answers. The simplest reason is that a great many tieflings are born to women expecting normal, human children. Fiends are masters of deception, and don’t always appear to mortals in their true form. And not every mortal carrying fiendish blood is a tiefling. Just as two brunettes might occasionally parent a redheaded child, fiendish blood can hide for generations before revealing itself in the flesh, the result of fiendish dalliances far back in the family tree.
Women who suspect they might be carrying a tiefling can have a difficult time determining the truth. Divination magic is notoriously unreliable when it comes to those who have not yet been born; results are cloudy and indeterminate in the best circumstances. In some cases, divinations have even indicated the unborn child is human, only to see the mother give birth to a tiefling months later.
Tiefling babes, even unborn, possess other resistances as well. Traditional contraceptives seem less effective against outsiders. Herbs and drugs used to induce miscarriages often fail to work. Spells require an experienced and powerful cleric in order to take effect, and many women lack the resources to find such a caster. Expectant mothers also rarely survive more drastic methods of inducing a miscarriage when a tiefling is involved.
Any preventive measure might prove moot if the mortal parent is the father and the tiefling is born to a fiendish mother who then drops the child on the father’s doorstep. In truth, fiends have no more desire to bear tiefling children than mortal women do. Gestation periods tend to be quicker for fiends, however, and they sometimes see the utility in having a tiefling child in the mortal world to act as a spy and informant. As fiends have little maternal instinct, they generally give birth to tieflings and then send them to live with their human parents- or any man willing to believe he might have once lain with a fiend.
Birthing a tiefling baby is unexpected for some women, and traumatic for almost all. Soon after birth, however, a mother realizes that bearing this child was the easiest part. The real challenge lies ahead.
Tieflings lack even a common appearance to draw them together as a race. It is said that no two tieflings look exactly alike; like fingerprints, each tiefling’s unique appearance marks him indelibly.
Most tieflings display all their unusual traits at birth, but some are born resembling their mortal parents only to develop their fiendish qualities months, even years, later. In other sad cases, an unborn tiefling develops characteristics so demanding on its developing system that it dies in utero, or survives only a shortly, horrific time after birth. In many regions, the appearance of a fiend-tainted child is reason enough to put the creature to death- and often the mother as well. In lands where the origins of tieflings are better understood, such births are mourned but don’t always end in murder. Human parents are not the only ones who shun tiefling children, though, as even hopeful tieflings, daring to wish for “normal” offspring, might condemn their newborns to terrible fates- abandonment, sale into slavery, or worse. The grim circumstances of tiefling births mean that many, even most, tieflings don’t survive the earliest days, a sorry condition that many pessimistic tieflings consider a blessing.
By the time they have grown to adolescence, all tieflings display at least one physical reflection of their fiendish ancestry. For the lucky, such traits prove so minor as to allow the tiefling to pass as human or a member of another exotic race. Most, though, are so clearly marked it is impossible to hide their nature without the help of elaborate disguises or magic. Fortunately, most tieflings develop supernatural abilities to aid them in their difficult lives. Many gain the ability to create areas of darkness- as if the spheres themselves were encouraging these beings to hide- but not all, with others expressing powers as strange and unusual as their nefarious ancestors. The dramatic changes a teifling undergoes are almost secondary, however, to the mental stress such a transformation brings.
Tieflings are typically strangers within their families, their cities, their societies- even their race, for many tieflings never encounter another of their own kind. Tiefling siblings are almost unheard of, as fiend-mortal couplings tend to occur only once. When tieflings appear in a family seemingly spontaneously, they are generally the only child to do so in their generation.
As a result, tieflings suffer from intense bouts of loneliness, self-loathing, and isolation. They are acutely aware that there are no others just like them, even among their own kind. Still, some seek out other tieflings, making it their life’s purpose to establish that they are not unique in the world. Even if such quests result in success, though, they rarely have happy endings: tieflings do no naturally bond. More aware than are others, perhaps, of just how strange and twisted they can be, they are slow to trust their own kind.
The isolation a tiefling feels grows as he realizes his own family- assuming they remain part of his life- are ashamed or even frightened of him. Some tieflings suffer childhoods of horrific abuse, relegated to tiny rooms, hidden completely from the outside world, bearing the brunt of the anger and the fear of their unprepared family. Little wonder most view everyone but themselves with intense mistrust.
Some tieflings blame their dark nature on the difficulties of their childhood. Even those rare few who grew up relatively unscarred suspect an unpleasant fundamental truth: that they naturally possess shadowed souls. A tiefling might work her whole life to throw off the shame of her heritage, yet still occasionally hear the seductive whisper of the power that could be hers if she embraced the darkness. While some might claim that all sentient beings face such temptations, for tieflings, this is almost a magnetic pull, like the hunger of an addict ever reeling from the depths of withdrawal. Even the most disciplined and virtuous tieflings find evil acts easy, natural- even enjoyable. Consciously embracing good proves difficult for tieflings, and no matter how long they travel such a path, the darkness remains like an infection they are unable to cure.
The level of this darkness and tieflings’ power to control such whims seems to vary from individual to individual. Some tieflings find it easy to ignore the temptation of their fiendish blood, while others find it nearly impossible. Many, of course, don’t even try, becoming creatures more akin to fiends than mortals. Such tieflings seem unable to go more than a few days without engaging in evil behavior, drawing perverse pleasure, even strength, from acts of deception, cruelty, or violence, seeking out opportunities to indulge their foul nature. For these depraved few, championing the cause of good is not even an option.
For those who prefer not to wantonly embrace their fiendish nature, life is an endless struggle, their internal world proving just as tumultuous as the world without.
To assume tieflings have a single society is misguided, as most seek nothing more than to meld into the human communities that surround yet typically shun them. In the rare instances when groups of tieflings congregate, they tend to adopt an unusual structure. Tieflings accept that the most human-looking individuals among them get the best treatment in cities and towns, and take it as no surprise that shopkeepers and guards treat obviously fiendish individuals with suspicion and hostility. Yet what tieflings rarely admit is that even those of their own kind treat “normal” –looking tieflings better than freaskish ones. To a creature told time after time that his appearance is his greatest drawback, it’s no wonder he would idolize a more highly valued appearance. On a more practical note, extremely conspicuous tieflings have a much harder life than others, and such experiences can warp their natures. Tieflings know that monstrous-looking tieflings quite likely act monstrous, a sad development arising from the way others treat them. Consequently, tieflings have an unofficial pecking order base on appearance- especially in civilized lands, where they often go to great lengths to appear human.
Due to their inherent mistrust of others, tieflings tend to have trouble with romance. Many consider it a source of personal pride to avoid entanglements of any sort; others take advantage of their natural, taboo exocitism and bounce from one superficial (and frequently abusive) relationship to the next, futilely seeking affirmation and acceptance. When tieflings do trust, they tend to do so with fierce loyalty. While a committed tiefling lover can prove the world’s most faithful companion, that devotion comes with a price, and tieflings rarely take the end of a relationship well. Unreasonable jealousy and feelings of betrayal boil quickly to the surface, turning passionate love into seething resentment.