At the start of my Pirates of the Aegean campaign, one of the players came to me with the concept of a Tiefling Druid. The problem was that there were no such things as Tieflings in the Fall from Heaven setting. My options looked like I could either tell the player there are no Tieflings in my Special Snowflake Setting so go make another character, or work it in. What was I to do? I dug into the lore of the setting and with a little elbow grease found a nice niche for tieflings.
In the end all that really matters is how you use them in your campaign. Being true to the lore is tempting but is often an unnecessary restriction on ideas. Realistically, is anyone going to care that I allow players to make Tieflings in this game with tons of great lore? The truth is a resounding hell no. So I got over myself and told the player, “Sure. Let’s come up with a good place to fit them in." I've found with enough creativity there's almost always a way to rationalize new ideas into established lore.
In my game I made the Tieflings the deformed children of the Sheiam borne in the year after the Stigmata of the Unborn. It seemed like a good fit. Here's something I put together (using Kael's comments) for that player in my campaign. It's the result of researching his people's past, and I wrote it in the voice of Elder Methyl (a religious scholar).
“Those called Tieflings are one of the rarest sentient races on Erebus, some argue they should be relegated to myth like the dragonborn as most have never laid eyes on one. I know for certain they are real, having spent several weeks observing their ways. Despite their devilish appearance, they live their day to day lives as most other races I’ve encountered. Yet the story of their origin is quite different from those of other races.
“Whereas every race has a creation myth, most explicitly state they were created by their gods. The Tiefling race claims they were made by demons, which is met with as much scorn as their appearance. However, they do not take pride in their creation myth and instead bear it as a curse. They tell of an event, called the Stigmata of the Unborn.
“It was the year 28 in the then new Age of Rebirth. There was once a city in which a cult of cabalists made pacts with demons. These demons then corrupted the city, filling it with such hatred and animosity that unborn children began to be possessed by infernal and unsanctified spirits. The marks of this possession were open gaping sores, horns, tails, and other physical deformities as well. The birth of these children aroused even more hatred and fighting among the people. Some were taken from their parents and killed, other mothers were blamed for the sickness and dragged through the streets. The name of this city has been lost, yet all accounts are certain it was located in what is now the Sheiam Empire.
“The children that survived became powerful leaders of the next generation. Their offspring also possessed many of the same deformities, commonly curling horns, thick tails, and pupil-less eyes. Eventually these leaders were overthrown and the offspring relegated to slavery. Some decades later there was a slave uprising followed by a mass exodus. They were harried by the armies of their former slavers, who gave chase due to the theft of a revered tome by the Tiefling elders. Adapting to the desert, the tribes were able to so harry their pursuers that they gave up the chase.
“The Tiefling elders then recounted their people’s journey north into the Desert of Myrh, where they wandered for generations in search of a prophesied ‘promised land’ which they claim they have not found to this day. The particular tribe I studied has said some of their community broke away, growing weary of the desert in their wanderings, and migrating to fairer climates. I imagine they are met with hostility and fear due to their frightening appearance. The resident Malakim population tolerates them, allowing the Tieflings to create their own communities as long as they do not infringe upon the Malakim’s own sacred lands and cities.
“My attempts to gain access to this tome, in addition to the exact nature of the prophecy and their promised land have been blocked. The Tiefling elders say it is only for a select few of their own kind to know, and that it is for my protection as well as theirs.”
You may have noticed a little inspiration from Lawrence of Arabia and the Book of Exodus. That’s because my player wanted to be play a Moses-like figure. More importantly, this creation required the application of the Second Rule of Dungeoncraft: Whenever you design a major piece of the campaign world, always devise at least one secret related to that piece. In fact this addition to the setting allowed me to create a couple of secrets. The first being where the promised land was and what the prophecy actually meant - not dropping that one yet. The second one was hinting at the true nature of the book, which was an ancient ritual to summon hell on Erebus. I was also able to tie the secret back to an element of the Civ 4 mod, and called the book “The Elegy of the Sheiam.” It created a great character arc where the druid became more and more obsessed with translating the book. At one point it was stolen from him and used, revealing it’s true purpose. A whole city was overrun by demons. It provided some really great material, and all because I let a player mess with my special snowflake.