Everyone has heard stories of those afflicted by fairy fire. It is a magical malady contracted by those foolish enough to go poking around in fairy business. It affects one's head. The victim sees lights, feels joy and mirth for no reason, and generally is distracted from the day to day things that ought to occupy a man's mind. And of course, if the afflicted doesn't go looking for more trouble, the ailment subsides in a week or two. Even children know these things.
But a much more serious condition can develop in those who meddle in the affairs of those more malicious fairy-kin, the imps. Imp fire will not just distract a man, it will drive him to ruin and beyond! How quickly the illness progresses depends greatly on the fortitude and initial goodness of the man, but it always follows the same pattern.
The first signs of the illness are that a man will become agitated, fidgety. He might display a short temper where he was always cool-headed before. Inside, he will feel a fire welling up in his gut. He will quickly be consumed by a desire, but not know what it is he wants. In an effort to satisfy this ambiguous desire he will begin seeking out the activities he enjoyed previously and, though they bring him no happiness now, take them to extremes. He may drink incredible amounts, light things on fire, seek the company of lewd women, or pick fights in back alleys.
By this point, the victim will feel warm to the touch and may sweat of appear flushed. He will become desperate, frustrated by the insatiable appetite growing in him. He will begin avoiding even his closest friends, devoting all his energies in the pursuit of the basest pleasures. Even a man who was previously good and mild is likely to commit horrible acts now. Murder, torture, rape are all possible and will be commited without regard to personal safety. Frequently he will become obsessed with the way things are consumed by flame and will be caught igniting churches, orphanages, and the like. The grotesque acts will continue to grow worse and worse.
If the wretched man has survived this long, he will be completely mad, having exhausted the delights offered by the sins. His appearance will be contorted, almost beyond recognition. He will howl and scream, completely unintelligibly. His body will shake violently, but his grip will be stronger than it ever was before. He will move about without purpose, destroying everything and everyone he comes across. Entire villages will be reduced to ash in his wake. He will no longer sleep or eat, he will simply destroy.
In the disease's final stages, nearly a month after the man began to act oddly, he will be a miserable inhuman thing. His skin will be dry, cracked and blister. His touch will burn hot enough to ignite wood on it's own. What's worse, the thing cannot be killed. Wounds, dismemberment, even beheading will go unnoticed. Now, what is left of the man will often seek out the densest areas of human population. It will run, stumble, crawl, or drag itself to the very centre of a large city. When it stops moving the air will be split with a sound like thunder and the carcass will burst, no longer able t contain the fury within. The force of such an eruption has been know to transform entire kingdoms into wastelands.
However, this disaster can be averted. A cure can be procured if the ailment is noticed early enough. If the victim's closest friends realise what is happening they can seek advice from a wise sage. Assuming the sage correctly identifies the disorder, a cure may be sought. A great many things have been tried, at on time or another, and a few of them have actually worked.
A deal may be struck with the imps. Something great being accomplished for them might be enough to persuade them to remove the curse. Of course, this might be quite distasteful as the imps will be most easily swayed by an act that furthers their own devious ends. Perhaps the entire fairy population of a forest will need to be terminated so that the imps might expand their territory. Then again, the imps might prefer that a king be assassinated and a country thrown into turmoil so that they might seat one of their own soulless puppets on the throne. It might even take something worse.
A god of healing or wellness may be beseeched to cure the victim. This will never be a simple task, though, as the deity will surely demand a great offering in return. A nation o nearby savages might need to be converted to worship the god in question. Or perhaps a plague will need to be stopped in his name before the god will help. In societies that have no god of healing, an ancient and forgotten deity might be sought out.
The cure might present itself in the form of a wondrously simple artifact. There is one story in which a man who had contracted imp fire was nearly saved when his friends found beautiful jeweled goblet that contained an endless supply of the purest ice cold water. Drinking deeply from the cup did not destroy the fire that burned inside him, but merely kept it at a manageable level for a time. The result being that the man controlled his terrible urges by driniking regularly until the man was mugged and his precious cup stolen.
Of course, any attempt at a cure will be complicated by the victim himself. At the same time that cure is being sought, the victim must be closely monitored. Even the most miraculous cure will be of little use if, by the time it is found, the victim has been found guilty of murder and madness and executed.