The Tieflings, as described in the PHB, have a rather vague origin: “a pact struck generations ago infused the essence of Asmodeus -overlord of the Nine Hells- into their their bloodline”. Here’s how I see this “pact” for SiT.
Justice in the City of Legacy has always been a straightforward affair. The Law is… unsophisticated. Even more so in the first decades.
Murder and treason led to execution. Rape, theft, fraud, everything else in fact, led to branding (or ear-cutting), the most common punishment. Those who were brought before justice who already bore this kind of unglorious identifier were instead banished for life from the city.
But it came to be known that most of these banished criminals were finding their way back to the City, sooner or later, often with the help of criminals inside that profited from the situation. They hid their brand with make-up, or they claimed they lost their ear to a beast, whatever. The whole thing was inefficient.
The Last Emperor sought advice, after all he wished his City to be exemplary. One, Cyrax the Lawgiver, a mysterious stranger coming from afar, convinced him on a drastic plan. The banished criminals would never be able to come back without everyone noticing. Their crooked mind would be shown to everybody, their very own appearance changed as to reflect it. They would have to stay away, or be killed, as they couldn’t hide or lie about their true self. The criminals would be “infused” with the essence of the fiends in a painful thaumaturgic process that would ostracize them forever!
The Mark of Infamy
Tieflings are bearers of the Mark, it has nothing to do with race. Humans, Halflings, Dwarves, Elves and Hobgoblins have been infused, transformed into the fiendish-looking Tieflings.
What wasn’t part of the deal proposed by Cyrax (Cyrax the Deceiver as he was later known) was that the Mark was inheritable, like a gene (and not always surfacing, it can skip many generations). Any descendant of one with the Mark may be potentially born a Tiefling.
It caused great distraught when this fact was known. Innocent infants, pure of heart, were born thusly cursed, destined to look like fiendish beings! The Mark and subsequent banishment, it was decided, would not be an option anymore. But for many, it was too late.
The Tieflings weren’t part of the City anymore, many of them were cloistered together, in a religious and troglodytic society. In a unfair reversal of their own fate, they didn’t allowed non-Tiefling to stay, even those born from them. And so, some of the non-tiefling infants were sent, or found their way, back to Legacy.
Half a century after the Edict of the Mark, with infants born with the curse both among the Exiled Tieflings and inside the city, the Last Emperor issued a Proclamation: all Tieflings would be made citizens.
Streets in Turmoil
The Tieflings have been around for more than two centuries and yet, old prejudices die hard. Many still see them as unholy degenerates, a view not helped by the fact that, some are just that.
Tieflings traits (homebrew variant)
I’ve dumped the fiery powers in favor of a more forked-tongue approach…
Choose another race for ability modifiers.
Cyrax’s Legacy: You are impervious to any magical attempt that would force you to tell the truth or detect if you’re lying.
In addition, once per short or long rest, you can use this feature to bolster a Charisma (deception) check and gain advantage.