1000s of years ago, demons and angels walked the land. They two groups never interacted much, but lived in peace for quite some time.

But at one point, humans became a thing, and a war between the demons and the angles broke out over how to handle the new, squishy group of beings. The angels wanted to let them roam and be free, the demons wanted to keep them secluded and away from the dangers free roaming would bring them. After many years of fighting, the angels were victorious, and the demons were forced to leave and made to live in their own country far secluded from the rest of the world.

Angry and vindictive, they swore to get revenge on the humans that caused them to be put away. So they stole some away, kidnapped them to their world and made them as slaves. Soon, a new generations of half-demons were born, and no one paid them any mind. They had human blood, but the blood of the demons was stronger than the blood of those mortals. Then another generation was born, and the blood was thinned even more.

While the demons payed no mind to this, the angels were getting worried. By this time the babies born looked more human than demon, most could easily pass, and they were worried if one of them somehow got out and mixed in with the human populace, trouble would start. So the angels made a new virus to stop the demons from breeding more with the humans, but it went horribly wrong. The virus ended up killing anyone with demon blood.

After it all was over, very few demon hybrids managed to survive, and they all vowed to never try to leave their home or to try to breed more within each other, not wanting their demon blood to grow any stronger. The few who were left made new towns and new laws, built a nice place and overall lived fine. Now, centuries later, the demon blood within them has all but worn away.

But it still lingers, very faintly, popping up at random turns. So a person with three eyes is just as likely to meet someone with grey skin, as a person who can turn invisible will accidentally bump into someone with tentacles for hair. Not too much, but a little demony is all.