Tiefling & Aasimar

Tieflings are a species that are created when a god or demon is somehow involved in the birth of a mortal child. They can be of any humanoid mortal origin.
The phrase 'tiefling' is most widely used, while 'aasimar' is sometimes used for those where it was a known, more benevolent entity. Cultural differences have a major effect on usage, with 'tiefling' often a derisive term for those with an antagonistic entity or unknown origin.
The exact nature of how a tiefling is born is heavily dependent on the entity that was involved in their birth. This can be a variety of situations, whether it is a god having a child with a mortal, a demon making a pact with their parent, or even simply that there were other tieflings in their ancestry.

Demonic Patrons

Arcane researchers still debate over why demonic patrons create tieflings, and what affect it has on the individual's soul after their death. Most agree that it is likely that a tiefling's soul is naturally drawn towards Abyssal soul wells, giving their patron easy access for use of their soul for crafting their demonic armies. Others purport this is untrue, citing the existence of tieflings in afterlife planes such as Eos.
All agree that it largely depends on the nature of the original pact. A tiefling's appearance may simply be a manifestation of the pact-holder's power, whether active or incidental, and a mark for them to find their creations again.

Basic Information

Anatomy

Tieflings and aasimar defy definition - they come in such a staggering variety of appearances that there are few categories to even fall under. Most take on the shape and general appearance of their mortal parent, as well as aspects of the entity involved in their birth. A tiefling with a halfling parent, for example, will share the same humanoid bodytype, height, and possibly some facial features.
The particular aspects a tiefling takes on vary. Most with demonic heritage have horns, unnatural hued skins, and even tails or cloven feet. Others have glowing eyes, strange odors, or other quirks. Those with more benevolent heritage tend to blend in better with their parent's society - they may be blessed with beauty, an appearance similar to the entity's servants, or others. Regardless, they tend to stand out, and face more scrutiny than most.
In rare cases, demon patrons have given their tieflings traits to help them fit in - in order to better manipulate mortals into trusting them.

Genetics and Reproduction

There is a very active community of researchers who are trying to chart and categorize the different types, and how their traits are passed down. Some individuals may have heritable traits, while others will have children without the aasimar and tiefling trait and appear as a normal individual of their grandparent's species.

Categories

Null - Individuals of this type do not pass their traits on. In some nations, it is illegal for a tiefling to have children if it cannot be guaranteed that they are a null tiefling - and such guarantees are rare.
Vorizi - Individuals of the vorizi type will pass their traits on to all of their children - all of their descendants will be tieflings. This type was named after the researcher who defined it, Rizin Alreath, and the demon god Vorozol who is known for his vast demonic hordes - though this type is found among many different origins.
Firstborn - Individuals of the firstborn type pass their traits on to the first of their children, meaning only one descendant is a tiefling.
Guth'taal - Individuals of the Guth'taal type skip generations, in varying amounts. Some Guth'taal skip every other generation, some skip more. The longest any Guth'taal has been confirmed to skip is once every 9 generations - the category is named after that particular family. It is presumed there may be others who skip even more, but none have been confirmed.

Jester by Merwild