Tieflings

Khora looked at the man from head to toe, warily taking in the sight before him. He wasn't one to turn away help based on appearances alone, but he'd simply never seen a man like this before. He mostly resembled a human, but his skin had lost much of its color, fading from brown to gray. His ears had elongated like an elf's, and split near the end, forming two tips rather than one. Horns sprouted from his temples, curving back towards his rear before turning around and pointing inward. A chitinous tail, leathery wings, hooved legs. His hair was going red at the roots, and clearly had been for months. "And you're not the only one like this?" the Earthshaker asked, making eye contact with his black irises.
"There are many more, at least seven hundred adults. I was the first, though. A proof of concept for Asmodeus, I imagine," the horned man replied, smiling softly.
The commander looked at him thoughtfully, contemplating the offer. This man had displayed power beyond even his own, and would prove invaluable in the wars to come. All he had to do was convince the world that a population of demon-kin were not acting on behalf of the abyss. Easy. "Remind me of your name again, comrade?"
The first tiefling smiled. "My people call me Xarkanas."

Conscripts

Before the Dratarian Wars, there were no tieflings. The forces of Hell have always been focused on order and balance, about maintaining the barriers between planes and protecting the nexus points and gateways between them. When Asmodeus took over after the death of Azrael, very little changed in regards to philosophy or laws. Sure, the entire realm got a new coat of infernal paint and everyone grew horns, but the offices and industry of Hell wouldn't stop for anyone to get used to it. This change in leadership and Asmodeus's past skirmishes with certain demonic figures is what eventually led to the Blood War, and the need for soldiers.
Many devils gave up their office jobs and took up arms to defend the realm while the more powerful Archdevils began to create new kinds of devils to bolster the ranks. Imps were reassigned from courier duties to flying in offensive swarms, Hellhounds were sired to accompany skirmishers as they broke through demonic frontlines, the first Merregon were pulled from the afterlife to serve as the expendable vanguard. This proved enough to hold the line, but as centuries passed, the paperwork began to pile up. It was Asariel that decided to turn to Thelekar for the answer.
No mortal could step foot in Hell for long without expensive and limited protective enchantments and items. Though it was certainly worth the gold to hire the strongest adventurers and mercenaries available to serve short stints on the frontlines, the Archdevil of Avernus wanted a more reliable source of skilled fighters, one that was replenishable without losing its quality over time. This led her to meet a man named Kane, a desperate human who was wandering Ardina to find every physician and witch he could to heal his son's sickness. The two struck an accord: he would willingly offer himself as a test subject so that Asariel could find a way to acclimate mortals to Hell, and she would give him the medicine needed to cure his son.
Working with her court, Asariel spent a year in her laboratory while keeping Kane's son in stasis, safe from death but not cured. The experiments on humans were going well, and these tests led to a great amount of research into the malleability of humans. According to her research, human bodies were simple and changeable, as if they were the base design and the rest of the mortal races were specialized versions of them. These experiments gave Kane tough, leathery skin, a set of impressive horns, claws, wings, a tail, and a myriad of other infernal mutations. Eventually, she had done it- Kane had become the first half-devil, the first tiefling. One man alone would not turn the war effort, however, and she needed more.
Interplanar law is very clear, and it is most strict where it is written, in Hell itself. Thelekar was the basis of existence, and as such, none were allowed to interfere in great capacities there. Those in Hell couldn't just take mortals and force them to fight for the greater good because of these laws, and so every soldier had to be a volunteer. Asariel offered Kane a second contract, this one much grander in scope than the first. She would render Kane's son invulnerable to damage or disease, if he would gather a force of willing soldiers and lead them in battle against the demonic invasion. Overcome with the possibility of protecting his son from all dangers, present or to come, he agreed, and eventually returned to a Hellgate with a few hundred volunteers. In return for their service, Asariel would house them, feed them, heal them, treat them well, and take care of their remains should they pass.
And so the group went to war, cutting across the demonic forces like a scythe through grass. The intelligence and ingenuity the Mortal Forces presented were mighty in the faces of a horde of mindless demons who were just as likely to take a bite out of their allies as their enemies. Time does not pass in Hell, and three hundred years went by before Asmodeus detected a change in the wind. War was brewing in Thelekar, a war that threatened to spill into all the realms should things go poorly. In an effort to shore up Hell's defenses, the Lord of the Nine personally traveled through hell, removing every speck of influence and Thought from Thelekar he could find, including the Mortal Forces. With only a few minutes to prepare and a basic rundown from Asariel, Kane and his people were dumped through a Hellgate which closed behind them, leaving the group in a world that had progressed without them.

A Partnership

With nowhere to go, the Mortal Forces grew roots at the foot of the mountain behind them, scavenging what little they could from the grasslands around them. The first buildings of their town were tents of demonhide, and their own food was the nutrition pellets they had managed to bring with them. Kane knew he had to provide for his people, and he only knew one way to do it: killing monsters for supplies and coin. His time in the Hells and his many mutations left him powerful, more powerful than even Asariel knew. During his stint in hell, the demons began to call him Xarkanas, the word for Fearsome in Old Infernal, and there could not have been a more accurate title. Rather than attempting to improve his swordplay, he had to actively weaken himself to avoid causing massive collateral damage to his surroundings.
Eventually, word spread of a settlement of demon-kin living in the Bloodhills, building walls and houses and wearing weapons and armor openly. At the time, the Bloodhills fell within the Sunfire Legion's territory, and with the rumors of a powerful demon warrior wandering from town to town spreading like wildfire, Commander Khora himself decided to get to the bottom of it all. He met with Kane and conversed with him, sharing their stories and the trials they overcame to get where they were now. By the end of the conversation, Khora had agreed to place the encampment under Sunfire Legion protection in return for Kane, now going by Xarkanas, joining the legion itself as one of his lieutenants. When Khora took over the legion after the passing of his predecessor, he named Xarkanas his commander, proof of the incredibly close friendship the two shared after years of fighting as comrades.
Thanks to the safety of Sunfire Legion endorsement, the tieflings had children of their own and greatly grew their settlement, which they named Greykeep, known in the fifth age as Yen’Lenora.

Draica

Most tieflings nowadays have little connection to their infernal past, instead considering themselves “Draican” rather than tiefling. The name of tiefling has instead grown to encompass a larger group of people, including those mutated and turned by demonic, celestial, and fey influence as well. It has become the greater name for the planes-touched, with each sub group considering themselves entirely separate from the rest, which isn't incorrect of them to do. An aasimar would not look at a demon-kin and see a brother or cousin, nor should they. It is simply the language used by scholars who want everything to be simple and orderly in a chaotic world.
Xarkanas’s specific bloodline have formed the grand Imperium Draica, a superpower on the continent of Arith that outnumbers the elven and human nations surrounding them by a substantial amount. Tieflings of different colors and horn types have gathered, forming small family groups over the years that have passed since their creation. They have mostly lost many of their more infernal traits, such as their hooved feet and wings, though some are still born with such appendages, such as the famous Raven of Voletaria, one of the last winged tieflings left on the planet.

Features and Traits

There are a number of different subgroups within the wider monicer of tiefling, all with infusions from different planes of existence. If none fit your specific character, you may choose the generic tiefling listed below. 
Ability Score Increase. One ability score of your choice increases by 2, and one ability score of your choice increases by 1.
Age. Tieflings mature at the same rate as humans but can live to be around 200 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 extra-planar 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.
Planar Resistance. Your blood has the power of another plane. You are resistant to Fire, Cold, Radiant, or Poison damage. 
Legacy. You know the minor illusion cantrip. When you reach 3rd level, you can cast the disguise self 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 invisibility 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. You can also cast these spells using spell slots you have of the appropriate level. 
Languages.  You can speak, read, and write Common and one language of your choice.

Related Ethnicities

• Asariel's Bloodline