Oni appears as a pale young man of maybe sixteen or seventeen. He has short black hair that is combed neatly back, and wears simple black clothes. He smiles all the time, and that, coupled with his red eyes, gives him a very frightening air.
There are always fell wizards who summon demons and bind them to their service, despite the risks involved. If a bound demon is not treated with the utmost caution, it will often escape and wreak it’s vengeance on the one who presumed to command it.
Oni is different, as demons go. He has never been summoned, and approaches his masters on his own initiative. He doesn’t care who he works for, or what their goals are. He is quite happy to take a low ranking position. Often, his master doesn’t even know what Oni’s true nature is.
The reason for all of this is that Oni’s greatest joy is causing pain, physical, emotional, and psychological. He regards torture as an art form, and himself as a master artist. He has centuries of experience in his chosen field, and knows how to cause the maximum amount of pain across the maximum period of time. Oni’s playthings don’t die slowly, sometimes lingering at his hands for years.
He has extensive knowledge of the psychology, anatomy, and physiology of human(oid)s, and his understanding of medicine is unrivaled. (He has, on very rare occasions, been persuaded to act as a doctor in addition to his normal pursuits, but those that ask him to do such a thing had damn well better make it worth his while, for their own sakes.)
Of course, inevitably, some hero will eventually come and destroy Oni’s master. When that happens, he simply leaves, moving on to the next despot, tyrant, madman, or evil wizard and offers his services to them.
Oni owns very little of his own, mostly relying on his current master to provide the equipment needed for his art. At most, he carries a well-cared for set of knives, scalpels, and needles. He perceives anyone who has to rely on actual torture-chamber equipment (such as racks, iron maidens, etc.) as lacking in imagination. Of course, he will happily use whatever he has at his disposal.
At any given time, Oni will have anywhere from two to thirteen "pets." These are people whom he has found to be remarkably resilient or hard to break, and he focuses his attention on them when he has nothing official to do, or if he’s bored. They can be distinguished from his ordinary victims because each of them will have Oni’s personal symbol carved into their chest. Depending on how long he’s had power over them, they will also exhibit various sickening injuries and deformities, and most are quite mad. Disturbingly, there have been a few pets that Oni has had for so long that they can’t live any other way, and will not leave him if they can help it.
He’s best used in very small doses. Torturing people is all he cares about, at all, and everything he does is in pursuit of that. Being a demon, he’s extremely hard to kill permanently, so he may pop up more than once. His true name is unknown: he gives whatever name amuses him.
He smiles all the time, is unceasingly polite, and creepy as hell. Perhaps the PCs need to rescue someone who’s fallen into his hands, or they’re just trying to take down his current master. He might even work in the dungeons of someone technically good- legitimate ruler, guardsmen/police, etc.