Tieflings are a race of part-human, part-hellish origin.
In the distant past, when humans met gods, they were faced with the latter's incredible power. Some prayed to them, flattering the gods so that their lives were spared. Some ran or hid in other lands. And some wished to take arms against them.
Seeing this, the demons offered the humans their blood. The mixture of human and demon blood gave birth to tieflings and possibly, arcane magic, which was able to kill gods permanently; hence the term 'godslayer magic'. This pact had one downside: the demons wanted their blood back eventually and would forcibly take it from tieflings at some point.
Physical factors Edit
In general, tieflings have a higher amount of talent and take to learning any skill naturally. This is thanks to the fact that tiefling blood awakens any sort of potential in its bearer's body. However, at a certain point, the amount of power it draws can no longer rise. It's at this point that the tiefling's blood 'ripens' and devils from Hell come for them to kill them and take their power.
Over time, tieflings have created a couple of solutions, which they communicate among themselves via distribution of a book on tiefling heritage. It contains a rune that transports whoever gives it blood into a room with an awakened tiefling (such as One), who can explain the origin of tieflings and present solutions to an uninitiated tiefling.
These solutions are either exorcising the demon blood, making the tiefling fully human or awakening whatever power there is in the demon blood - as One puts it, 'taking from it instead of it giving to you' - and then fending off any hellspawn (or other tieflings as well) that come for the tiefling.
Tieflings are also able to live out their lives without reaching their full potential, but devils will still 'collect' them after death.
Awakened state Edit
Awakened tieflings are those that have taken the second option - drawing power from their demon blood. This process is usually extremely agonising and few tieflings survive it. It requires a tiefling to 'harden their humanity into a weapon' so that the awakened demon blood does not completely overtake them. Upon awakening, their physical appearance is supposed to change in different ways.
They are described as having power matching that of spiritfolk and their weaknesses differ.
As the tieflings descended from humans, their nature supposedly differs just as much. There do exist prejudices based on the fact their blood is partially demonic, but it's unconfirmed whether the demon blood has any effect on a tiefling's personality.
Tieflings aren't currently known to possess any lands. There is, however, the kingdom of Tannhauser, which Markus hails from. In Tannhauser, tieflings - the Velafi family - act as servants to the royal - Tannhauser - family, which is of Aasimar heritage. Though it is presented as the way it has always been, the Tannhauser family has actually overtaken the throne about a century ago and masked it as 'an unsuccessful attempt at revolution'.