The origin of fiends

Dark gods, potent evil spirits, the souls of the truly wicked- fiends may be any of these things, all of them, or none. What people do know is that they plague mortals and are available to help those who invoke them with correct offerings, though doing so is strictly forbidden in nearly every culture. The story of the first fiend is one from the Níłtsą́ Bikąʼ people of the Great Continent is perhaps the most popular version of where these wicked things came from, and has been copied into slightly different forms in several cultures.
Thousands of years ago, when humans first awakened under the stars and dwarves still fashioned their first tools, a man named Bahaagi walked among his people and served them as an elder, offering counsel and healing when needed, for he was among the very first humans to awaken and knew more than all born after him. Bahaagi tirelessly sought to improve the lives of his community, and spoke at length with the spirits of the world around them, passing on lessons to his people on how to live harmoniously with the land and its resources. For years, Bahaagi's tribe lived an idyllic life, hunting and fishing to supplement the gathering of other foods, playing music in their settlements and playing games in their abundant spare time. A nearby peoples had grown in number however, and war touched Bahaagi's tribe for the first time. The people coming against them had wicked weapons made from obsidian and hardwood; the hunters among Bahaagi's people weren't accustomed to war, and many delayed their arrows against fellow humans, while others wielded spears as though they fought a bear, not a cunning human warrior. The battle was one-sided, and as the attacking warriors whooped in triumph and carried off baskets of plunder and the hands of their slain opponents, Bahaagi counted the dead and found his two sons lying among the corpses, hands missing. Consumed by grief, he fled the mourning village and plunged into the thick forest, desperately seeking some meaning, some sign from Kytheria herself that this too existed within the natural order. Nature could indeed be cruel, however violence of this sort, of this magnitude, could not be found among the beasts of the forest, even those who toyed with their prey before consuming it. Bahaagi wandered aimlessly for days, unable to sleep or eat, his anger and hatred solidifying within him. One day he came upon a mountain lion chewing on the entrails of a deer still alive, and bleating with terror. Bahaagi beheld something then that he'd never see before; a new spirit had emerged from the deer's dying breath, one not of nature or elements but of pain and fear. This tiny spirit glowed fiercely before Bahaagi's eyes, and despite how weak it appeared, it inspired Bahaagi, who nurtured the growing darkness within him.
The village of Bahaagi hadn't seen him for a full week, and still coped with the loss of so many young men and women when their elder returned. Their spirits briefly lifted, the villagers began crowding around Bahaagi, though all present began to murmur and shift uncomfortably upon seeing how his eyes shone with frenzy, and their hope swiftly turned to fear. Bahaagi saw the small spirits that had grown from the suffering of his wounded and dead kin, and spoke to them all, channeling his hatred and rage with words into the tiny spirits. The villagers had no defense against what occurred next, for Bahaagi had long been their sole source of spiritual guidance, and the spirits possessed what villagers they could and turned them upon each other. Each possessed person slew any villagers who had escaped and ate their hearts, drinking in the spirit made by their grief and growing stronger; when only the possessed remained, they tore each other apart, consuming hearts and spirits with reckless abandon. The last cannibal standing turned to attack Bahaagi himself, but too late; Bahaagi had held onto an obsidian dagger form the battle, and buried this in the last cannibal's chest. He then ate the heart he dug out and he, too, drank the final spirit produced by this grisly ritual, affecting a transformation into something that had never walked Kytheria before. His footsteps burned the ground where he walked, and his eyes glowed yellow and orange with flame. Smoke rose from his skin and his countenance, while still mostly human in appearance, gained a horrific presence that would paralyze all but the strongest willed. His preparations complete, Bahaagi followed the trail of the attackers back to their village, and the horrors he visited upon those people are not to be spoken of. Nature had nothing within her that matched human cruelty, and that day Bahaagi filled the void with blood and pain; from his new home built of skin and bones, he fashioned the first of the fiends who would follow him, spirits of pain fattened on the continued suffering of the village's inhabitants. Thousands of years later, there are fiends too numerous to count, but it is said that whenever one summons a fiend to bargain or demand a favor, revenge will always be taken up by the summoned evil, though the results will almost always be more than the summoner can bear.