This is a great manhwa. The main theme of this manhwa is mystery and it executes it very well you don't know what is gonna happen next and it makes you feel that sense of mystery for a long time. Not every mystery manhwa accomplishes to do this feat so good. Sometimes when you are reading a mystery manhwa you know what is gonna happen next but not in this one.
A great mystery Manhwa, reminds me of a movie called 'Time Lapse'. I recommend it if you are into that kind of mistery/time-play kind of stories.