This is the 1st time i'm writing a review (sorry for my bad english)
I think this drama is QUITE GOOD and have great potential to be top drama but lost the steam when reaching the final episode.
STORYLINE is GOOD and i love it. (Although, there's a lot of people say that, this is a copycat of Goblin. Not at all, its totally different.) The thing that i HATE is the unnecessary scene, the drama duration is too long, so they add too much of unnecessary story.
ACTING is 10/10. Lee Seung Gi, Oh YeonSeo, Cha Seungwon & all other characters really save the show. I watched until the final episode because of their great acting skills.
MUSIC/OST: The OST IS GREAT. If you don't plan to watch this drama, i PREFER you to listen to all of the OSTs (Seriously, its Gold but very underrated, idk why)

WEAKNESS/LACKING:
1. Too much unnecessary scene (Might be because of long duration)
2. WORST Cinematography/CG EFFECT
3. The finale episode is slow

My advice is don't compare this to other fantasy drama, watch first then judge