Tieflings are the progeny of those who have made deals with dark forces to gain advantage in life. Ignorant people believe them to be inherently Evil descendants of demons and devils, but in reality their soul is mortal and free to choose between Good and Evil. They have powers inherited from their dark mark and are blessed with an innate cunning.
There are shadows in every corner of the world. If you know the right signs and rituals, you can get their attention. It is in this way that mortals can strike bargains with dark forces for immediate power and advantage.
Bargains with forces of darkness are common. Mortals, driven by ambition, do not know how to negotiate with evils and thus are taken advantage of. Promising a first child, or their own body, or their next of kin to the Shadow gives rise to Tieflings. It marks all progeny, gifting them with some infernal-like abilities but otherwise leaving their soul free and human. (Note: Warlocks are among the few mortals able to negotiate with evils without inadvertently serving them).
Tieflings have been marked by the Shadow. Their births are marred by the Umbral Wind, and while their souls are still Mortal (and can thus choose between Good and Evil), their appearance is impacted. Even noble human hearts can be shaken by the sight of the Tiefling: no matter how open minded or how Good they are, the Tiefling appearance is designed by the Currents to invoke ancient fears and grudges against demons and devils. Those fears are as innate as standing before a grizzly bear or swimming with sharks. They are intuitive.
Unfortunately, their appearance creates a vicious cycle: the Tiefling’s appearance creates suspicion among their peers, who then ostracize the tieflings, who then must resort to unscrupulous means to survive, thus creating further suspicion and hatred. The mark of the Shadow is how evil undoes the Good in people’s hearts: by preying on their fear of the unknown and driving them to commit evil acts.
The Oaths that govern Tieflings give them unique abilities. Most tragically among them is that a Tiefling can constantly see and hear the opportunities the Shadows could present them. It is present in the magic they weave from their heritage and in their ability to see through Darkness: Humans have to go looking for Evil. Evil approaches Tieflings constantly.
Some Tieflings embrace the Shadow, constantly dabbling in the powers presented to them. They become powerful criminals, allying themselves with others to become smugglers, thieves, or even assassins. Others reject those powers completely and strive to overcome the sins of their ancestors.
Almost all humans have seen a Tiefling, and most know one or two. Ignorant communities that harbor prejudices around Tieflings often relegate them to physical work or maintain mistrustful trade agreements with them. In more cosmopolitan locals, their innate intelligence and charisma makes them talented deal makers and negotiators. They are famously popular among wordly halfling sailors. In major cities groups of Tieflings are found in pocket communities (typically near docks), where they share in their unique burdens.
They also possess a few small islands of their own. Some are reclusive hideaways, while others are more welcoming to outsiders. The most famous Tiefling island is The Horned Bank. Carved from an entire single island, this vault and money lending facility is the most secure in the world. Only Tieflings are permitted in their employee, and their funding efforts have raised up and brought down empires. A hub of the world economy, The Horned Bank is the exemplar of Tiefling acumen.