The 8th Night asks you to absorb a lot. To its credit, the film does everything it can to catch you up and provides a familiar structure - stop the monster, save the world - to make it easy to follow. Even so, I recommend you read a bit of the synopsis on Wikipedia (the first two paragraphs should do). With a bit of a lead start, you’ll be able to follow with no problem.
2,500 years ago, the Buddha stopped a monster by ripping out its eyes and hiding them in faraway lands. If reunited, Hell would be unleashed. After discovering the eye, a disgraced professor becomes its vessel and begins fulfilling its prophecy. He is the first “stepping stone” as the eye progresses toward eight victims over the course of eight nights. To stop the end of the world, monk Chung-seok (Nam Da-reum) searches for former exorcist Park Jin-soo (Lee sung-min). Meanwhile, the eye’s trail of desiccated bodies draws the attention of homicide detective Kim Ho-tae (Park Hae-joon).
To simplify it, a monk and an exorcist who’s lost his faith try to stop an evil spirit from ending the world. See what I mean about the familiar story? It’s the right move to make when you’re dealing with weird mythology. I say weird, but I mean it in a good way. The monster's eyes being different colors and containing a sentient evil in themselves, it's legend and prophecy, the tasks it has to accomplish to end the world, the way it can be stopped all feel like something you would find in an ancient temple. The convoluted conflict makes it interesting and feel “real”.
Details throughout make this story believable as something happening in the middle of a much bigger world. The eye’s final victim must be a/the Virgin Shaman (Kim Yoo-jung). Jin-soo’s job is to kill her before the eye possesses her body, which will make humble Cheong-seok really uncomfortable once he finds out. Their interactions are usually kind of sweet, adding a human touch to a story that threatens to be overwhelming. At the police precinct, no one knows what’s happening. This means two guys looking for a woman while dead bodies pile up look mighty suspicious. This makes detective Ho-tae an unexpected, but human (in several ways) obstacle.
Several twists and mysteries are unlocked along the way, so plot-wise and character-wise, The 8th Night is strong. The horror element is lacking, however. On that scale, this is just a notch above something like Ghost Rider. The characters rarely interact with the evil, and the eye’s victims we only meet as they’re about to be possessed. When they are, the results are so-so. Sometimes, seeing a monster smile can make it extra menacing. This time, it looks goofy. This wouldn’t be such an issue if the movie was shorter but it almost hits the two-hour mark. That’s a long time waiting to be spooked.
The 8th Night is perfect for watching at home. You can freely rewind it if you need to remember a critical plot point and the fact that you’re watching it “for free” will offset the lack of scares. The interesting mythology, story, and characters will then tip it over the edge into "glad to have seen it". (Original Korean with English Subtitles, July 20, 2021)