At some point in their lineage, possibly a parent, a tiefling is related to a Devil from the Nine Hells. Their unnaturally coloured skin and eyes, their horns and their tail; it all hits to this infernal lineage. At for many, this is all they see when they encounter a tiefling, and so the race is often faced with distrust and racism. When everyone expects you to be a bad person, it's sometimes easier to live up to the stereotype than it is to constantly fight it, even though there's nothing inherently evil about tieflings.
For the most part, tieflings stem from the union of a human and a devil. They take on most of the characteristics of a human, but they also have horns, as well as a thick tail that is between a metre and a metre and a half long. Their skin can be almost any colour, but often some shade of red or black. Their eyes usually don't have an iris or a pupil, instead, they are onle solid, slightly glowing, colour.
Genetics and Reproduction
A tiefling is the result of fiendish blood in a regular humanoid bloodline. And for the most part, a tiefling’s reproduction resembles this (probably human) ancestry. While their horns are small and slightly soft at birth, anyone who has given birth will tell you that a couple of extra bumps on the head will make the experience worse. Some tiefling produce heat while in the womb, others have other peculiar effects.
Perception and Sensory Capabilities
Different tieflings can inherit different abilities from their infernal blood, but all of them can at least see in the dark.