I almost didn't watch this one because of the bad reviews. Glad I ignored them. This is a prequel to 1 & 2. The end of two asked "why are there so many demons in the mountains" and this goes into answering that question for us. The creators also wanted to explore another Taiwanese folk tale or urban legend while at the same time building off what they've already created. It was a great idea, it made the film feel fresh while still looking for ways things will end up tying into the first two films. We get a lot more backstory on the mountains, the Tiger Temple introduced in part 2 and the early life of the father of the missing girl in part 2 who's also the Tiger Lord. Characters from the first film appear in the credits and after credits scenes so stick around for those. I hope this encourages more people to finish the trilogy. A review complained about the cgi but it looked like the same quality as the first two. No idea what they meant by that. I also hope they continue the story at some point while exploring more folk tales or urban legends. Really enjoyed this series.

Firstly, the deity that the main character worships is the Tiger Deity and it is an actual thing in Taiwan. Because spirits usually latch onto someone when they do bad things, the deities will also require a representative in this world. The deities usually pick the person who they want to represent them and it's usually a family thing. So the grandfather was worshipping it. His son, the protagonist, is also its worshipper, and now that he's dead, it's pretty obvious that the son will be following in his footsteps.

The part where everyone finds corny (when the son wakes up and walks into the shrine like a tiger with his eyes glowing) that's the Tiger Deity manifesting itself. That's why when the son realized his father was going off to fight the evil spirit, he asked him not to go and stay close and he would protect him. It means the son knows and is aware that the Tiger Deity has picked him as his representative. But the father never realized because he has been aloof from the deity all this time.

The part where the protagonist was in the forest with a few followers who were all wearing tiger gear and he seems to be praying, he was actually asking the Tiger Deity to manifest itself (which it never did) and we later realize it's because when his wife died, he blamed the deity for not helping her and his resentment actually prevented the deity from manifesting.

There's actually 2 spirits at play in this story. The sick boy and the evil mountain demon. Usually, more powerful beings would just consume lesser spirits to make themselves more powerful, but it seems like that the demon has somehow accepted the sick boy as a disciple of some sort.

The pianist mother didn't die, instead, she's living within a cocoon that the sick boy has created. It's obvious the sick boy just wanted family to take care of him. It's pretty sad.

On the other hand, the evil mountain demon is pitting itself against the Tiger Deity. The protagonist actually doesn't really know that the evil mountain demon is out and about because that's not who the demon is after. The demon is actually after his son, that's why the boy keeps on dreaming about the demon. And at the end we realize it's because the Tiger Deity has already picked the boy to be his representative.

Anyways, that's my understanding of the movie. I come from Taiwan and I've listened to so many stories about people being haunted and needing to go to a shrine to have it taken care of that I can appreciate the movie a lot more. If I had watched this movie a year ago with no knowledge, I would not have understood it well and would have written it off as an okay movie with cheap jump scares. But, yeah, ghosts really do suddenly appear out of the blue to scare people.