Fine Young Women Who Kill People
Tieflings are born as a curse on Human families who have made a pact with a demon. Many generations ago some Warlock dudes made pacts with various gods and monsters to gain extra magic powers. As part of this Faustian arrangement, some Demons decided to curse the families of these Warlocks with Demon-Blooded children. Those children are the Tieflings.
There is a shame associated with their presence. If you have a wacky Tiefling uncle in your family it’s best to hide him lest it cost your daughters their rich fiancees.
The birth of a Tiefling into an otherwise respectable family is a scary moment. Many women choose to pretend the child was stillborn then go leave it in an orphanage or spike pit. Don’t try to burn your Tiefling baby as they resist 5 fire so you’d have to cook it for a few days before it starts to singe.
You can’t even fool your meddling neighbours into believing your Tiefling child is normal with a baggy hat and dark glasses. It will look like Hellboy and occasionally set things on fire. Meddling neighbours are right on top of that sort of thing.
Sadly, few Tieflings ever experience a proper, loving, biological family. There are some monastries and orphanages who take in baby Tieflings and hope to foster them out to families but they’re rarely successful. Tieflings often end up as gypsies, tramps or theives, making their living by tricking or seducing Humans out of their money.
As a race, they’re largely similar to Humans but due to their outcast status have become adept at sneaking and lying. Their skin is often bright red. How that helps with the sneaking thing is anyone’s guess.