The Demon Xarlocc was, like many demonic beings, deeply involved in the internal politics of Hell, where souls were the demonic equivalent of money, and the demon who controlled the most souls had a lot of respect from fellow demons. There were several ways to get hold of souls; as a gift from another demon, but these were rare, as demons did not give gifts lightly; through gambling but Xarlocc was no good at that; through stealing from a Greater Demon, but the penalty if caught was the death of one’s demonic body and the everlasting torture of one’s demonic soul. Or through wheeling and dealing; it was demonic lore that a soul that one managed to persuade one’s owner to hand over to you was your own in Hell.
Of course, the thing was, most humans were wise enough not to sign over their souls in the first place. Xarlocc thought of a way around this. He targeted the dying and the terrified of death, and he promised them that they need not sign away their souls to get his help; all they would need to do was to sign a promise to Be Bad.
In return, their bodies would be healed of any diseases or illnesses and filled with demonic energy, making them slightly stronger then a normal human would be, as long as they preformed a Bad Deed each day. It could be something illegal that broke the law in some way, something immoral but not necessarily illegal, like telling people lies or seducing their loved ones behind their backs, or just something unpleasant, like sacking someone from their job. Like any shylock, he was ruthless to those who neglected their part of the bargain for even a day; their bodies would start to rot. At first it would be scarcely noticeable; a smell of something foul, then their eyes would start to film, their bodies start to swell and go bad.
Unless they committed something really evil…a murder, a rape or something of that level, then within a matter of days their bodies would rot to the point that their brains died; and it was near certain that the Bad Deeds they had done in the past would guarantee for them a place in Hell. Should one of them do something Good, then they would start to decay until they either died or did an equal or greater Evil act, when the decay would be healed.
Some Xarloccians cannot handle what they have done and try and go straight, or end up in solitary confinement in prison; these swiftly become horrifying rotting corpses, desperate in most cases to commit murder so as to return to their normal forms before their bodies and their souls plunge into Hell. Others have perished on the gallows or by the sword after being caught committing one crime too many by law enforcement or by vigilantes.
Still others have joined crime syndicates, where those not murdered by their fellow criminals in internal purges or turf wars have often risen high in the organisations, and a handful of the most determined and enterprising have formed large corporations and pay their dues to Xarlocc by treating their workers badly, selling weapons on the black market or even encouraging politicians to start wars, as a really nasty war can in effect pay Xarlocc for decades. It is possible that some of the nastiest wars on the planet have been started so as to give Xarlocc what he wants; and the souls of the evil who die in such wars flow straight into Xarlocc’s accounts as if he had personally recruited them, making him one of the richest demons of the Nine Hells.
Since they know that should they die, they are effectively as hell-bound as if they had signed away their souls in a more conventional fashion, they will fight with no quarter. A decomposing Xarloccian cannot see far, as his or her eyes are a milky white, nor hear all that well, but the demonic strength remains to the very end, whilst one that is paying his or her dues will have all their senses enhanced greatly and will be a fearful foe. However one dies, it's body will swiftly decompose down to the bare white bones.
What a Xarloccian gains from the deal are three things, provided they keep their side of it. Increased strength;twice the strength of the average human. Immunity from the vast majority of diseases in the world. And old age will not claim them. They can still die violently or in an accident, however,not to mention what happens to them if they try and break the deal or are unable to see it through.