Tieflings in Mystara
by James MishlerWere I to run Mystara using 4E (and I have no plans to), here are two options I would use to add the Tieflings:
#1: Fitting into Current Continuity with Minimal Ripples
The tiefling race is not native to Mystara; they come from another planet. They were brought here in drips and drabs over the last thousand years, in numbers only in Alphatia but also elsewhere, brought as servants to the powerful wizards through mighty spells. Some stayed over time, and so there were always small tieflings families here and there, but again, never in great numbers, and usually feared, especially by religious folks.
More were brought in, on both Alphatian and Thyatian sides, during the Colonial Wars in Alasiya. Following the end of the wars with both sides being kicked out after the rise of Al-Kalim, the Alphatian tieflings went to the Isle of Dawn or back to Alphatia, while the Thyatian tieflings went back to Thyatis City or went to the Highlands, where they fought as mercenaries on all sides, and thus after the founding of the Principalities could be found throughout the land of Glantri.
Thus through 1000 AC they were uncommon in Alphatia and Glantri, rare in Thyatis City and Thyatian colonies, and very rare elsewhere.
Then came the Wrath of the Immortals (if you follow that line). Alphatians, Glantrians, and Thyatians summoned armies of tieflings, none of which tipped the balance of the battles in any significant way as all summoned equally (only the Heldannic Knights resisted the temptations of the summonings, as it was not their way). Also, when hard-pressed in the east, the Huleans summoned armies of tieflings, which would have tipped the balance had not the summonings gone awry and half or more of the tieflings escaped control and turned against their erstwhile masters. Following the Wrath of the Immortals, many Alphatian tieflings, already out "in the field" on the Isle of Dawn (being preferred mercenaries) survived. In Darokin many of the "rebel tieflings" settled down with the approval of the Darokinians and became regular members of the community. Same happened everywhere they were summoned, so now there are tieflings in most nations of the Known World. Many tieflings live in Glantri, Darokin, Thyatis, on the Isle of Dawn, and on the smaller ruined isles of Alphatia and further east. Many also settled in Karameikos, where they have more open room; perhaps there is even a new tiefling domain there. Tieflings are still quite rare in Ylaruam (where they are considered evil creatures due to many serving, however unwillingly, the colonial masters), Ethengar (where they are hunted like humanoids), the Five Shires (though some live in Shireton), and the Northern Reaches (where they simply have never really expanded, finding the locals to be too savage for their tastes). Some evil and depraved tieflings have settled in the Broken Lands with the idea of making it a homeland for evil tieflings. Some Alphatian tieflings who served in the "wet navy" have settled in Ierendi and Minrothad. There are also many tieflings still in Hule and occupied Sind.
So after the WotI, tieflings are known and found throughout the Known World, and are not an unexpected sight.
#3: Retconning them into Mystara
This is what I would do if I were "re-building" Mystara essentially from the ground-up as a 4E setting, using the materials presented in 4E as a building block for new Canon.
Tieflings are descended from nobles of the Taymoran Empire who turned away from undeath and sought power through infernal alliances. Sheltered from the worst of the infighting between nosferatu and vampires, and with their stronghold in the northwestern territories of the empire, after the fall of Taymor they rebuilt a remnant of their lost empire in what is today Darokin and named it Bael Turath, ruled by those with the infernal bloodline.
Timeline
2000 BC... Kingdom of Bael Turath founded, expands north to the Glantrian Glaciers, south and east to the Altan Tepes, and west into Sind and lands of the Azcan Savages. Capital is at the modern site of Akorros.
1800 BC... First battles with expanding Nithian Kingdom. First battles go to Bael Turath, which settles the lands south of the Altan Tepes
1700 BC... Bael Turath weakened due to energies from Blackmoor Device, lose southwest to Tahkati Stormrider and his Children of the Horse; the Children of the Viper are descended from tieflings who remained behind, and today have many tieflings among their clans (most other Atruaghin clans despise tieflings for this reason).
1500 BC... Second major series of wars with Nithia go to the Nithians, with their Antalian mercenaries. Mix of tieflings, their Taymoran subjects, Antalian mercenaries and Nithians settle in the lands south of the Altan Tepes.
1400 BC... Cursed by Bael Turath, the peoples of the lands south of the Altan Tepes revert to stone-age savagery (they are later assisted by the Hutaakans and the mixed peoples become the Traladarans; tieflings were very rare among them and usually live on the outskirts of their civilisation).
1300 BC... In the midst of decline, the western lands (Sind) break away from Bael Turath.
mid-1200's BC... The beastmen invade from the west, conquering Sind, the Atruaghin Clans, and threatening Bael Turath itself, which has fallen into decadence. The various nobles of Bael Turath fight against one another, often using humanoid tribes as mercenaries. This civil war continues on for more than a century.
1010 BC... The tiefling civil war ends, and the divisive realms of Bael Turath are again ruled by a single king. He then builds a grand army to scourge the lands of the independent humanoid tribes and subjugate any survivors. This great crusade backfires when the formerly dis-united humanoid tribes ally and, after a decade-long war, destroy Bael Turath and its capital in 1000 BC. Humans and tieflings are reduced to slavery or savagery in the region of modern Darokin. Many tieflings flee to Nithia or the Traldar lands; thence on to Alphatia when that realm is settled.
1000 BC... The Beastmen arrive in Traldar lands on the heels of the tieflings; this does not help their reputation among the later-day Traladara, as they believe tieflings are evil, and so the few that live there continue to be on the edge of society.
800 BC... Tiefling and human slaves of the humanoids flee into the lands opening up with the retreat of the glaciers from the Highlands. They are left there, as they have little or nothing the humanoids want.
700 BC... Around this time the tieflings of the Highlands found a kingdom they call Bael Turath, but it is never anything but a shadow of their lost kingdom. Unfortunately, as the glaciers continue to recede opening up new lands, this budding kingdom comes into conflict with the dragonborn kingdom of Arkhosia, high upon the Adri Varma Plateau (which has been cleared for some centuries).
ca. 600 to 400 BC... Mixed clans of Kerendans and Hattians, and small numbers of Thyatians and Traladarans, begin to re-settle the lands along the Streel River in modern Darokin (the Eastwind were descended from a mixed Kerendan/Hattian clan). They slaughter all the orcs and other humanoids they can find and absorb the surviving humans into their own clans including the tieflings, though they remain on the edge of society.
300 AC... After many long and futile wars against the dragonborn of Arkhosia, both the New Kingdom of Bael Turath and Arkhosia fall into chaos, their realms a patchwork of petty principalities and humanoid tribes descended from mercenaries.
350 AC... The lands of the fallen New Kingdom of Bael Turath are devastated by plague, killing 90% of all Taymoran-descended humans and 50% of the tieflings. When the Flaems arrive in several decades they find the land a virtual wilderness, with only small settlements of humans and tieflings...
And from this point basically pick up the tale from #1, above, but the tieflings weren't summoned to Mystara, they were native to Mystara, with their numbers growing greater in the century before the WotI due to growing peace and prosperity all around in the lands they settle. The Hulean tiefling forces were also native to Hule, descended from tiefling slaves brought west from the Known World centuries ago by various humanoid tribes.