Stick with it. The 8th Night gets better as the movie progresses.
It starts out a little pedestrian. At this stage I was thinking that perhaps it would be another uninspiring demon movie; but as the story slowly develops, the atmosphere improves. There are an increasing number of creepy scenes and good uses of cinematography that give rise to some unease.
As with many Asian horror movies, there aren't many jump scares. Well, that's not what Asian horror is about. That's saved for rehashed Hollywood. This movie tries to create an uneasy, tense atmosphere, and partially achieves that. Had it achieved it more, the movie would have been more rewarding.
However, there are many good aspects to this movie. Good cinematography, good acting (that on occasion would come across better with improved editing and/or direction), a story that delivers enough twists and developments to keep the viewer intrigued, and a steadily building pace.
Whilst not being the greatest example of Asian horror, The 8th Night is far from being the worst. Don't expect a masterpiece. Do expect a decent movie.