The Cirran Stratocracy ruled our world from the skies. Their floating city of Zebulon could travel anywhere, and see everything that transpired on the surface world. They thoroughly dominated the surface dwellers - thoroughly, but not completely. When the wolf backs the rabbit into a corner, the rabbit loses nothing by fighting back.
The rebellion began small, as rebellions do. There had been other resistances, other movements, other prophets of victory, other brief sparks of hope in darkness. This group was different. They must have no weaknesses to exploit. There must be no fear, no hesitation. Comforting illusions and high principles were luxuries they could not afford. They had allies within the Cirran aristocracy, but these would be treated as resources rather than friends.
The movement retreated underground. If the Cirrans ruled from the sky, they could have it. None of the arrogant cloud-lords would dare to descend into the Tartarean depths; their pride would not allow it. The rebels had no such compunctions. They hollowed out tunnels, blocked off passageways, established secret routes, opened ventilation shafts. A child could be born and grow to manhood without seeing the sun. Many did. Darkness was the price of freedom.
The conspirators obtained secret Cirran mechanistry, including soul-control and body-changing mechanisms. They dove into feverish research, sacrificing themselves to be test subjects or to prevent the Cirrans from learning of their work. The product of this heart-breaking work was a process that both physically and magically transformed the subject.
One by one, the rebels had their souls forcibly stripped away. Their minds became even harder and more callous, if that is possible. Their bodies were transformed into inhuman fighting machines, scaled and clawed and hideous. Unable to feel even regret, the transformed rebels knew only that death must come to the Cirrans.
The demons do not tell me if they were responsible for the fall of Zebulon. There are secrets they still keep, even from their chronicler. I will say only that Zebulon fell - or rose, if you wish - and the Stratocracy came to a rather spectacular end.
The conversion process was irreversible. A demon, once made, could not be returned to a human existence. This, too, was a price worth paying. As living creatures, they had unearthly resilience. As fearless rebels, they had ironclad determination, the only emotion left to them. They could not end their own lives, and they were too strong for others to overcome them. They remained in their caverns, and still do, unable to do anything but suffer.
Well, almost anything. The Cirran soul-control mechanistry still functions, and they still understand its workings. Those foolish enough to bargain with demons without a full understanding of the price do indeed lose their souls. A demon is able to temporarily bond with such captured souls, and through them experience love, pain, anguish - the full spectrum of human experience. It is their only outlet and their only solace. Even a few hours in a soul machine is a lifetime of comfort to a human trapped forever in a horrid shell and monstrous mentality.
Demons will bargain with their summoners, but they will not be the cause of cruelty. They will take cold and relentless revenge, if asked to do so by a petitioner. They will stand guard over important places, nourished through eternity by their summoner’s soul. There are even a few brave scholars who offered their own souls, asking for nothing, knowing the mercy such an offering would bring.
There is indeed a Hell, as the common folk say. It is indeed populated by demons, as they whisper. It is a prison to prolong the suffering of the damned, as they say. The tragedy of Hell is that these two groups are one in the same.