I'm trying to remember the title of a relatively low budget horror-comedy that I watched somewhere in the last decade. I'm not sure how much of the comedy was intentional, but I recall finding some aspects funny.
A man and his wife/fiance go to a secluded house in the woods. She is having mental health issues and ostensibly it is to help her recover. (In the attic?) She finds a (blue?) demon mask. I think it was a Japanese style demon, i.e. oni.
She may or may not have some visions when she finds the mask. Soon after, a figure wearing the mask shows up trying to kill her.
We find out the masked figure is someone paid by the husband to kill her because divorce is too hard or something. All the visions are in her mind.... or so we think.
The next twist is that the demon is actually real. It possesses her and kills the husband after luring him to bed.
It was likely made post 2003 or so.
Most of my Google searching has revolved around "blue demon" but it has been unsuccessful.
In response to the given answer, I'm fairly certain that the two main characters were white. I would be surprised if it was based off any Asian booksl or game.
I had some suspicion that the film was an After Dark horror film. I ended up just looking through all of them and found Nightmare Man which is the movie I was looking for. As expected, I remembered some of the details wrong, including that the mask was an 'African fertility mask', not that of an Oni.