Origins of Fiends

At the dawn of time the first gods created the demigod-like Nephilim to walk their creation, in the form of both giant and dragon. In time, so great was the divine potence of the universe, aspects of their first gods began to develop independence from their being. These aspects became the most powerful of celestials, the paragons. Paragons created angels to act as their messengers and envoys from the outer spheres to the material plane. Then an event occured. The exact nature of that event is lost to time but it saw one of the gods unmade, a second gravely injured and a third driven mad with despair. The outer planes were marred by the formation of the Abyss. This led to the first Dragonform War as the gods took sides and turned the prime material plane into a battlefield. At the height of the Dragonform War, a plan was hatched by the Pale Dragon and it's followers to turn the tide of battle to their favour. Some of the celestials were in favour of this plan. Some were not. Two of the most powerful of the Pallite generals fought in an apocalyptic combat over this course of action - Orcus and Ysatan. Ysatan was slain by Orcus on the fields of Ammar and Ysatan's second in command, Asmodeus, took his remaining loyal forces and withdrew from the prime material plane. The remainder of the celestials loyal to the Pallite cause mustered on the Cerulean Fields to assist in an epic confrontation that would see the Pale Dragon slay Arcyne, the God of Magic, stealing from her a single profane word of power in the Major Eldyrion, the language of creation. The Pallite plan come to fruition, the Pale Dragon uttered the Profane Word and the endless dead of the thousand year war arose under the command of the Pallite forces. It looked as if the war was won. But the Pale Dragon for not anticipate the intervention of the Yrdragon, the eldest god of creation, who throughout the war had remained dormant. The final two dragonform eggs hatched, and Selyne and Sullyste, the twin gods of the sun and the moon, were born, and laid down the endless dead. The Yrdragon made of their flesh both the Celestial Scale, to protect the prime material from extraplanar threats, and the Seal of Selyne, to banish the mortal pallites back to the dark below. And all immortals were driven from Dyrn. Those angels who had been loyal to the Pallites became fiends. Ysatan loyalists retained the ultimate beauty and grace of the celestial form, and became devils. But those angels who had remained loyal to the Pallites were twisted and deformed by their exposure to the Profane Word. They became demons and fled to the abyss. Those mortals who had pledged service to the various celestials who had taken part in the way were variously blessed by them. Aasimar and Tiefling bloodlines has their origins here, in the blood soaked fields of the Cerulean Fields, as the gods and angels and demons fled the world and the remaining giants declared the ordning and began the Age of Paucity. Trapped behind the celestial scale, the various fiends fought their own wars against each other and against the celestials until in time, as fiends are wont to do, they found a loophole. They could not willingly pass the celestial scale, not physically. But they could still whisper to mortals, influence them with visions or dreams. And ultimately, to cross over completely, although they could not do so willingly, they could be invited. Or summoned. All it required was a mortal who was willing to forfeit their soul to do so. The benefit of this to the fiend was twofold. One, it allowed them a physical presence on the material plane, where they could corrupt souls directly without needing to resort to whispers and lies in the night. And secondly, a mortal soul so forfeit would pass to the demon at the moment of death. That person's afterlife would begin as a mane or a dretch in the service of the entity they had sold their soul to. This was highly desirable for a fiend as either a way to grow more powerful in the abyss, or to move up the hierarchy of the hells. Of course if possible many fiends so summoned would try to shortcut this process immediately and kill their summoner but some, especially devils, might be inclined to keep their summoner alive a little longer, making use of their catspaw to further their influence within the material plane. Tieflings and Aasimar, by dint of their ancient heritage, were an exception to this rule. These souls were marked from birth by a powerful celestial or fiend. When claimed by a celestial, such souls would find immediate admittance to the upper spheres as a newly born angel, without any need to await purgatory or limbo. When claimed by a fiendish power, however, their fate was mirrored in the spheres below - instead of living our their eternity as mane or a dretch they would be fated to enter the afterlife as a much more powerful entity. As such, these souls were especially desirable to fiends. Aasimar and Tieflings have an innate understanding of this, although it affects them in different ways. Aasimar are compulsively called to serve a celestial power. They innately know their time on the material plane is short and they inhabit a transitory form on the way to the upper spheres.
Tieflings on the other hand have an innate realisation that their mortal souls are doomed. They know that the essence of their being is too valuable for any fiendish being to pass up and that at the moment of their death, there will be a tumultuous squabble by fiends of all types for their soul. For some this may lead to great piety - "Perhaps if I serve a god faithfully enough it will claim me and save me from the hells." For others it leads to more questionable morality. "My soul is condemned anyway. Why shouldn't I practice a little necromancy? Tieflings gain from their heritage an innate ability to sense the secret motivations of those around them. This affects their psyche in often unpredictable ways, and many tieflings may display divergent morality influenced by knowing the secret fears and yearnings of those around them. This leads on to a new racial ability for tieflings within the world of Dyrn. I haven't added anything to Aasimar as their abilities already feed in well to the lore I have in mind for them and also they're already a pretty strong racial option. Detect Motivation
The fiendish heritage in your Tiefling bloodline gives you an inherent ability to sense the motivations of those around you. Tieflings who trace their heritage back to Asmodeus and the Nine Hells can sense the desires of those around them. Tieflings who travel their heritage back to Orcus and the abyss can sense a person's greatest fears.
As an action, a you can attempt to divine the fears (if your bloodline is abyssal) or desires (if your bloodline is infernal) of a creature within 30 feet. The target must make a Charisma Saving Throw against a DC of 8 + your proficiency bonus + your charisma modifier. On a success, you learn one fact about the psychology of the target - either a deep seated desire or a deep seated fear, depending on your fiendish heritage.
A target who succeeds on their save is immune to this ability for the next 24 hours. Whether they succeed or fail, the target is unaware you have attempted to detect their motivations in this way, although they do feel a sense of unease in your presence. You can take this action a number of times equal to your proficiency bonus. You regain all used of this ability on the completion of a long rest.