Well I got a couple different worlds so shall do each in brief:
World 1:
Only "Tieflings" exist in this world, however they are very different from traditional Tieflings. They are called "Nefas" and have more in common appearance wise with Aberrations then Devils.
A Nefas is the child of two human parents who's child while in the womb is warped by the fluxes in reality that cover this world. As such it is born twisted and different, these changes can be subtle or alarming.
Nefas are looked down upon, and many are killed at birth. However, they are accepted within society given their numbers, the randomness of their births and many high-level members of the Aristocracy are Nefas (given that many Noble families are in close proximity to the ruling Aberrations or "Pandorans").
World 2:
Both Tieflings and Dragonborn reside in this world, they are two of the three most common races with only Humans being larger.
They each rule over their own empires. The Tieflings rule over a empire strikingly similar to the Roman Empire with the debaucheries of it cranked up. The Dragonborn empire is similar to the Egyptian, however Dragons have taken the place of many of the gods.
Humans are the common people, they are merchants, slaves, farmers, etc.
World 3:
Both Tieflings and Dragonborn also reside within this world. This is a world that has been shaped by the Feywild, specifically through dreams and nightmares. For the Feywild is a realm where such rituals of the mind take form and shape, as Humans began to delve into the Feywild the borders between the worlds began to break down and dreams and nightmares began to flood into our world. Some are invisible and cannot be seen, others are all too real.
The Tieflings are children born of mothers who are haunted by nightmares (many Tieflings are born at times of strife, war or disease). Though they share none of the mental traits we exhibit with nightmares, some begin to embody it as a way to identify with any kind of past.
Most Tieflings hide their devilish appearance (this is a common motif in the nightmares of humans and thus become the common form of Tieflings), and try to live normally. Many however given the stigma surrounding them and superstition seek work away from others.
Dragonborn are another race spawned from the thoughts of dreams and nightmares. No one is sure when they arrived in the world, only that they spawned from the dreams of Dragons. They share many common appearances with Dragons but are quite different in nature.
They have embodied many of the knightly ideals dreamt about by humans, but their monstrous nature has left such ideals to fester into mercenary profit. They are amongst the best mercenary forces in the world.