One of the first Flayed Ones was a man named Oliver Cotton Chapman, an American expat living and working in North Africa. He frequented disreputable places and became what the locals of Tangier describe as a Predatory Poet, the sort of man who in his non-work hours frequents opium dens, and the brothels that offer more than women, searches after mysteries and antiquities, and who invariably end up deranged or end their own lives.
A Flayed One looks like a man or woman who has had their skin removed. They glisten moistly, and leave a pinkish residue everywhere they go, and on everything they touch. They have no hair, about a quarter, the more viciously skinned, do not have eyelids or lips, their faces much more skull like than wet and fleshy. They have all been de-sexed, the reproductive organs removed from their bodies.
Biology note - the Flayed Ones are no longer bound by the constraints of mortality. They do not eat, drink, sleep, excrete, dream, and so forth. They stay constantly moist because that is their nature, and lacking skin they would seem susceptible to illness, disease, or injury, but they are supernatural and resilient. Despite their appearance, intelligence, and activity, Flayed Ones behave similar to high power undead.
Insensate - The Flayed Ones are all but numb, unable to feel pleasure, pain, taste, scent, or any other stimulus. This isn't complete, they can feel when something touches them, or if they have been shot, or electrocuted. This renders their existence maddening, as most Flayed Ones were sadists, masochists, sensualists, thrill-seekers, and more.
Empathy - there is a loophole, the Flayed Ones are able to feel the sensations of the people near them. This leads the Flayed Ones to carry out acts of extreme violence, depravity, and perversion on those they catch, because to feel anything is ecstasy to the numb.
Obfuscation - Most people are relatively accepting of a Flayed One so long as they are dressed. The finer the clothing, the more accepting people are, ignoring the gore and red slime they leave behind them. This is a limited ability, and works best in dark places. A Flayed One will avoid bright light, crowded locales, and such. They prefer one on one contact as this allows them to control the level of sensation they experience.
Undead Status - Flayed Ones cannot be mind controlled, cannot suffer critical hits, require no sustenance of any sort, and so forth, the same as normal undead.
Oliver Cotton Chapman was a dark magician. He had come to the Interzone seeking after magical power and relics, and he found them. He was already a student of the occult, and an accomplished magician when he arrived. What he found granted him great power, and he sought deeper and darker mysteries, until he delved too deeply, and awoke something terrible. It took Oliver Chapman to its own hellish dominion where it ripped his skin from his body, tortured and abused him, and showed him terrible things. Had his sanity been intact at the time, he would have gone mad. But he was insane long before arriving in Hell or Africa. Chapman escaped, and from his horrors learned to channel the magics of the inchoate and overwhelming Hell he had been incarcerated in.
Oblivious - a Flayed One, long inured to numbness and exploiting others for sensation, has no empathy or sympathy. This makes is relatively easy to deceive them, or manipulate them, assuming the person in question has the strength of will to attempt it.
Indebted to Hell - a conjurer or summoner who is willing to take the supreme risk of summoning an infernal power like a demon or devil will find that they will be quite excited an amenable to hunting down a Flayed One and bringing them back to the infernal realms.
The Flayed Ones are antagonists. They are manipulators and abusers, people who are obsessed with power and control over others, and seeking immortality. As such, they can serve as powerful foes, using magic in largely mundane settings, supernatural foes in modern set games. Defeating them requires wit, access to magic items, trickery, and almost never brute force.