In contrast to most forms of demon or spirit, the Bjonak is not found in any sort of free state in a place where it can interact with mortals. Instead, the Bjonak is tied to a mask. The demon is normally trapped within, dormant. When a person takes ownership of the mask, however, the Bjonak begins to feed upon them. It hears their secret wishes and desires and materializes from the mask, acting upon these whims with the utter ruthlessness which only a demon can muster. Gradually, it increasingly takes on their desires, and they are reduced to a shadow of their former self, until, eventually, they are drawn into the mask and the Bjonak can escape and replace them.
The Bjonak resides within a mask for most of its existence. Normally, this mask has a distinctly inhuman appearance, though this could range from inhuman ugliness to bestial features to something abstract or impressionist. As the Bjonak grows into the person it is feeding upon, the mask slowly takes on their features. This is a slow process, often unnoticeable by the victim, though someone who is not familiar with the mask will be struck by the resemblance.
When the Bjonak emerges from the mask, it has a generally human form, and is easily mistaken for a person. The only giveaway is its face, or lack thereof. Instead, it is simply flat where it should have a face, as if someone cut off the front of its head. As the creature replaces someone, this slowly grows into their face, but until it actually replaces them, this looks more like their face trying to push through a sheet or some putty, and it is not effective as a disguise.
While the Bjonak exists inside its mask, it is in some ways aware of the world around it. It is in this way that the Bjonak latches onto a person and learns about them. In a teenage girl's bedroom, it might be able to read her diary; in a medieval castle, it may overhear people gossiping; in the far future, it might be able to read the chip in someone's head. But it is generally not capable of direct mind reading or empathic understanding. It is also capable of learning from other sources than its target - if it knows the girl it is feeding on has a crush on a certain boy, then its ears will perk up (metaphorically) if it hears other people talking about him going to prom with someone. Ultimately, the demon seeks to learn the desires of the person it is targeting, then will materialize to act upon them.
The Bjonak can materialize fairly regularly, but it will generally only materialize at night, when it is easier for it to hide its nature. It can only materialize one body at a time, however, and if this body is injured, it must rest within the mask to recuperate. Furthermore, it is unaware of the mask's surroundings while material.
Once materialized, the monster pursues the aims of its target without scruples. If it is feeding on a girl with a crush, it may attack or try to scare off her rivals, kidnap the person in question, or steal things to impress them. It does have some sense of scale, and may try less drastic measures like intimidation before moving to something like murder, simply to try to stay on the down low.
While in material form, the Bjonak is exceptionally strong, and a vicious combatant. It is also unkillable - if slain, it merely dissipates from existence, reforming inside the mask. The demon, however, is incapable of speech, or really, any sort of communication beyond the most basic point and grunt. The demon is capable of harming the person whom it is feeding upon, but will only do so if in danger - if they are killed, the demon can no longer feed upon them.
As the Bjonak accomplishes deeds, it and the mask grow towards the person they are impersonating, and the Bjonak gains an increasing understanding of the person's context and surroundings - that is, it gains an increasing ability to work out what it should do next.
Eventually, the demon will replace the person upon whom it feeds. To do this, it simply needs to act upon the person's desires enough to fully take on their appearance and whims. It doesn't need to meet with any sort of ultimate success, though some amount of limited success is required for the appearance change to occur. Once the mask matches their face, the person fades from the world and is trapped in the mask, and the Bjonak is free, and takes on the exact appearance and abilities of its target, including speech, basic personality, memories, and so on. But it retains its warped version of their desires and its utter ruthlessness. To others, they may seem to become a monster overnight. Of course, that's pretty much exactly what has happened.
Defeating the Bjonak
How can you kill something which just returns to a mask when it is slain. Well, obviously, you have to destroy the mask. This is not as easy as it sounds, because you have to deal with the demon, too. If the mask is broken, the demon is instantly released, and is very angry. While it will eventually die without the mask to retreat into, it can do quite a bit of damage before this happens. But if the demon is slain once the mask is destroyed, then it is truly dead.
Once the demon replaces someone, it can be killed just like they could have been, but the replaced person is gone forever.
Origin of the Spirit
There are two essential possibilities for where Bjonaks come from. It is possible that, when a person is replaced and drawn into a mask, they are eventually turned into a new Bjonak. Alternatively, perhaps they are conjured by wizards or strong emotions, and are simply trapped within masks.
A teenager has a Bjonak living in his or her bedroom. It is attacking bullies at school, romantic rivals, and disliked teachers or principals. Just the job for a monster hunter and her buddies, or a renegade Time Lord, or....
The family of an important noblewoman is being murdered. The culprit is a Bjonak impersonating her husband, trying to make her into an heiress so that he (it) can inherit her fortune.
A genius scientist is the Bjonak's target. It performs the darker experiments he dreams of but would never do, or perhaps arranges the happy coincidences of dead bodies he needs for his research.
A wicked sorcerer lets a Bjonak feed on him, using it to deal with his enemies, and intending to destroy it before it destroys him.
All the clues in a mystery point towards one person - only they had a motive. But it's actually a Bjonak feeding on them.
A player character gets a sinister shadow - it seems like trouble follows them everywhere they go.