Homebrew Aasimar Tiefling Species Details

I saw him, running to help us, his head blazing with celestial light, but his hands wreathed in infernal flame. Fire blasted from his hands even as shafts of pure light lanced from his eyes.  I knew then that we were meeting a new being, one heard of in legends but never seen until now.  An unholy blending of Aasimar and Tiefling, combining the best and worst of both races.

— Feren Kindal, My Travels and Trials

Aasimar bear within their souls the light of the heavens. They are descended from humans with a touch of the power of Mount Celestia.
Tiefling carry within them the darkness of the depths.  They are descended from humans touched by the essence of Asmodeus.
An Aasimar/Tiefling Hybrid carries both of these within them.  Their conflicting natures are often at war with each other.

Hybrid Bloodline

- Tieflings are derived from human bloodlines, and in the broadest possible sense, they still look human. However, their infernal heritage has left a clear imprint on their appearance. Some Tieflings have large horns that take any of a variety of shapes. They sometimes have thick tails, four to five feet long, which lash or coil around their legs when they get upset or nervous. Their eyes are solid colors—black, red or gold—with no visible pupil. Their skin tones cover the full range of human coloration, but also include various shades of red or even purple. Their hair, cascading down their back, is usually dark, from black or brown to dark red, dark blue, or purple.

- Aasimar are placed in the world to serve as guardians of law and good. From an early age, an Aasimar receives visions and guidance from celestial entities via dreams. These dreams help shape an Aasimar, granting a sense of destiny and a desire for righteousness. Each Aasimar can count a specific celestial agent of the gods as a guide. This entity is typically a deva, an angel who acts as a messenger to the mortal world. Aasimar are a people of otherworldly visages, with luminous features that reveal their celestial heritage, although they are much closer to the human norm than a Tiefling.  They share the trait of solid color eyes, although they tend to be lighter colors.  Their hair is usually light colored.

- An Aasimar/Tiefling Hybrid is a melding of both those races and can show physical signs of either or both of those races at the same time.  Some will display their Tiefling heritage with horns, a tail and unusual skin and eye coloring, while others display their Aasimar heritage.  Most will display a combination of these.

Hidden Wanderers

While Aasimar are strident foes of evil, they typically prefer to keep a low profile. An Aasimar inevitably draws the attention of evil cultists, fiends, and other enemies of good.  When traveling, Aasimar prefer hoods, closed helms, and other gear that allows them to conceal their identities. The secrecy they desire is never worth endangering the innocent.

Lacking a homeland, Tieflings know that they have to make their own way in the world and that they have to be strong to survive. They are not quick to trust anyone who claims to be a friend, but when the companions of a Tiefling demonstrate that they trust him or her, the Tiefling learns to extend the same trust to them and once an Tiefling  gives someone loyalty, the Tiefling is a firm friend or ally for life.

The Aasimar/Tiefling Hybrid typically shows most of these characteristics.  They almost never settle in one place, they find it extremely difficult to make friends or to trust any other race.  They prefer hoods and other gear to conceal their identity if possible.  They do not share the acceptance a pure Aasimar experiences in some areas, nor the grudging tolerance shown to pure Tieflings, but instead become very much alone, except when among their own race. 
However, they can become intensely loyal, almost devoted.  Convincing an Aasimar/Tiefling hybrid to join you will be an exercise in patience, but once they have committed, they will never betray any of your party for any reason and will probably sacrifice themselves to save you.

Aasimar/Tiefling Hybrid Guides

An Aasimar/Tiefling Hybrid, except for one who has turned to evil, will usually have a link to an angelic being. That being—usually a deva—provides guidance, although this connection functions only in dreams. As such, the guidance is not a direct command or a simple spoken word. Instead, the hybrid receives visions, prophecies, and feelings.

The angelic being is far from omniscient. Its guidance is based on its understanding of the tenets of good and evil.  The angelic being must also strive to balance the dual nature existing within the Aasimar/Tiefling Hybrid.  For this reason, the Hybrid will often mistrust the guidance and disregard it completely.

Aasimar/Tiefling Hybrid Names

Hybrids are born from human parents, and they use the same naming conventions as their native culture.

MET WITH MISTRUST

Ignorant people tend to be suspicious of tieflings, assuming that their infernal heritage has left its mark on their personality and morality, not just their appearance. The reality is that a tiefling’s bloodline doesn’t affect their personality. They are gifted with magic from the infernal realms but chart their own course in life.

Aasimar Tiefling Traits

Tieflings share certain racial traits as a result of their infernal descent, and Aasimars share certain traits due to their divinely touched blood.

Hybrid Racial Spells (Spellcaster Only)

As a result of your unique heritage, spellcasters gain the ability to cast 2 extra spells as they gain in levels.

At Level 3 you learn Lesser Restoration.
At Level 3 you learn Burning Hands.
At Level 5 you learn Flame Blade.
At Level 5 you learn Daylight.

Both these spells use Charisma as the spellcasting ability.
Both spells require you to use one of your spell slots to cast, but neither are part of your known spell lists.

Alignment

Imbued with both celestial and infernal power, an Aasimar/Tiefling Hybrid can vary wildly, depending on which (if any) of their heritage gains the upper hand.  
Most will tend to Chaotic behaviour as the two struggle for ascendancy, and they will usually be either good or neutral, but very rarely evil.

Ability Score Increase

Your Intelligence score increases by 1, your Constitution score increases by 2, and your Charisma score increases by 2.

Age

Tieflings mature at the same rate as humans but can live up to 140 years old.

Size

Tieflings are about the same size and build as humans. Your size is Medium.

Speed

Your base walking speed is 30 feet.

Darkvision

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you 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.

Other Worldly Resistance

You have resistance to fire damage, necrotic damage, and radiant damage.

Other Worldly Legacy

You know the thaumaturgy cantrip. You know the light. 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 Common, Infernal, and Celestial