Eidolons and Daimones (integrating 5.5e Aasimar and Tieflings)
by Marc SaindonWhile basic D&D avoided christian mythology, notably demonology, to spare itself from the Satanic Panic of the 80s, I paradoxically do appreciate the lack infernal-celestial elements in Mystara, as it allows it to be a unique setting, different from FR or Greyhawk in that way.
With that said, 5.5e includes Aasimar and Tieflings as core species now, and rather than say "no", one possibility is to tap into greek and roman mythology instead of judeo-christian lore. The entities we culturally perceive as "angels" are not biblically accurate (the OGs are lovecraftian entities), but a recycling of greco-roman "winged victories" artwork. Thus, by keeping things roman, we can reimagine the Aasimar as the "Eidolon", hybrids of humans and the spirit attendants (Genius) of the Thyatian Immortals. They would still be rare, and often associated with temple life and various religious sanctuaries of the Empire.
Their darker counterpart, the Daimones, are not evil per se, but still associated with the Underworld (perhaps "the Hades" is Thyatis's Shadowfell if we retcon that concept into the setting). The three legacies (Abyssal, Chthonic, Infernal) are rebranded along the lines of a Furie maternal ancestor: Alecto, Megeara, Tilphousia. You could have an "Alectan" Daimon instead of an Abyssal Tiefling, for example.
The Furies play a role in dispensing justice in Thyatian religion, and while grim and dangerous, they are not the same as truly evil demons of christian lore. The Daimones would still be a rare sight in Thyatian society, not necessarily shunned, but still causing some unease due to their otherworldly origin.
Neither species would have animosity towards the other, as they are rooted in polytheist roman mythology rather than the more manichaen confrontations of middle eastern religions.
notes:
Genius (mythology)
Eidolon
Daimon
Erinyes
Image from: https://upload.wikimedia.org/wikipedia/commons/4/4b/Winged_genius_Boscoreale_Louvre_P23.jpg
