It had everything ... and did so little with it. First two episodes - great. 3 & 4 - okay.
It was fine up until the last two episodes.
Plot development was lame. No surprises, no twists, nothing to figure out, just setup, and then failure.

No character development. No new discoveries. The heroine, bless her, never won my respect.
She was a mess, no self confidence, nothing heroic about her. She was a weak character.

Putting the little girl in a compromising position - and the whole cult thing was a red herring. Absolute crap.

Johan's character and background were shamefully underdeveloped.

Poor Kim Nam Gil was forced to just act miserable throughout the series. It's not a wonder he welcomed death.
I wanted to die too. Honestly, halfway through the last episode, I couldn't wait for it to end. But wait ... there's more! ?

A terrible waste of something that could have been a great story, with great actors, in awesome surroundings.
But, unfortunately, the writers were asleep at the wheel.

If they make another one (as they threatened to do at the ending) I won't be watching it.
After all the hype ... there were like 200 monsters at the end, against 5 people. Totally unrealistic. Then, in that already unrealistic situation, they weakened their main character, Van, into a virtual liability.
The best thing in the ending was the old priest showing up with a silver bullet rocket launcher.

Abysmal ending to an okay story.