Tiefling

Tieflings are conceived in one of three ways.
The first occurs when a Demon possessed human has a child with a mortal. These Tieflings look like the two mortal parents with tiefling traits.

The second occurs when a Devil in its mortal form has a child with a mortal. These tieflings looks like a mix between the mother and the mortal form of the Devil, with added Tiefling traits.
The third occurs when a Tiefling has a child with another mortal. The resulting child looks like a mix between the two parents. The Tiefling traits stay true from generation to generation, never diminishing. Every child of a Tiefling is a Tiefling.
Tieflings can be born of any race and thus don't have any unifying culture, however, a curious side-effect of their nature is an inborn understanding of the Infernal tongue. Every Tiefling has a different experience based on the situation they are born into. One might face severe oppression and rejection by even their own family, while another is fully accepted or even revered.

Tiefling Player Traits (For D&D 5e)

Ability Score Increase.  Your Intelligence score increases by 1 and your Charisma score increases by 2
Age.  Tieflings mature at the same rate as their mortal parent, and die at approximately the same age.
Size.  Tieflings are about the same size and build as their mortal parent.  You are the same size as your mortal parent.
Speed.  Your base walking speed is 30 feet.
Darkvision.  You can see in dim light within 60 feet as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
Hellish Resistance.  You have resistance to fire damage.
Infernal Legacy.  You know the Thaumaturgy cantrip.  When you reach 3rd level, you can cast the Hellish Rebuke spell as a 2nd-level spell once with this trait and regain the ability to do so when you finish a long rest.  When you reach 5th level, you can cast the Darkness spell once with this trait and regain the ability to do so when you finish a long rest.  Charisma is your spellcasting ability for these spells.
Languages.  You can speak, read, and write Infernal and one other language, usually the language used by those you were raised around.