I don't know why I watched this - I dislike horror as a genre and it needs to be exceptionally well made or innovative for me to enjoy. This was not exceptionally well made - but I know why I watched - Park Seo Joon Ssi!
The plot was fairly standard. The only unusual element was the ML "atheism" which I enjoyed in so far as it existed at all.
The demon was OK if predictable - seemed very standard and actually pretty dull
The lighting budget was extremely low. That seemed very predictable too
None of the characters were well developed, and, while there were opportunities for psychological exploration they were not followed through. I think if there had been more exploration then the frankly manufactured fear could have become much stronger. None of the battles seemed real because they were kinda preplanned.
In addition, from the very start I thought it a shame that the "possessed" were depicted exactly as if having a particularly frightening schizophrenic episode which - before they were understood to be a mental illness and not the embodiment of evil - led to many desperately ill people being tortured by various church authorities (and this still happens in certain cultures). It made me deeply uncomfortable, and the writer/director could have done much better.
I watched this to the end, but frankly fast-forwarded a good 30% of it.
The good things - the acting was excellent from all involved. I thought the SFX were good. Some elements of the design were very pretty - PSJ's wardrobe for example, his car, his bike; the demon's bar looked pretty cool, and the logo was stylish. Trying hard here - hope you appreciate the effort.
If you like this kind of guff, you may enjoy.