Imagine if you will that your eyes do not see everything as it is. Among you are those who are not quite the same, yet not all that different. That lady has red eyes, or that man is wearing a jacket on a hot day. These small things and more to a trained eye signals a demon.

Demons of reality are much different than the demons of mythology. Many are friendly and happy to leave others alone, though not all. You see all demons start out as humans, just humans that are "special". Their demon soul can manifest its self and transform these humans into true demons, and this is how they are born. 

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Ok so here is the basic idea. Humans are a neutral vessal that has no soul, most of the time. However, if a human is the descendant of a demon-human pairing they receive a soul. This soul remains mostly inactive (causing some health problems and temper issues but nothing life threatening early on) unless "unsealed" by a manifested demon. When "unsealed" the human body is reformed over the course of weeks to months (depending on how dramatic) to the form of the soul. If a soul is not unsealed eventually the human body will degrade until they die a painful death when they are middle aged to late, often of organ failure.

Some humans do have the ability to sense a demon, often they are descendant of demon-human pairings but to far down the line to have a demon soul themself (the souls stop appearing after about 3 generations. Also demon-demon pairings cannot produce children as women are infertile).

The general roleplay would start out with 1 demon and 2+ demons-to-be all working at the same place. Trust me I have a few plot twists before transformation time. There are types of demons and I can put of their information if this takes off.