If based this manhwa : a person who died on the mountain harboring a grudge or regret so his soul was held captive by the mountain god
But if you want to know the true meaning of changgui (no need to read the part below if you only care about this manhwa)...
spirit of someone killed and eaten by a tiger, to break free from the curse's shackles and move on to the afterlife, the Changgui forced to assist the tiger in its hunt by luring and trapping other humans as new victims (though the author clarified that they modified this slightly: the character dies from a fall but still becomes a Changgui, as being eaten by a tiger on a mountain is unlikely in modern times).
The term *changgui* might not be used in the official English version. I mention it because it is used in the official Korean version.

I’ve finished reading up to the final chapter (Chapter 27+Author's note) because I couldn't bear waiting three weeks for the conclusion, lol
(below this are spoilers)
Chapters 25–27 were so sad.
Geonpil finally left with Beom.
Yeonsung ended up becoming a changgui, so sad!
We also got a glimpse of the Mountain God's face. I'm so curious!
He looks like that will be covered in a side story later. Maybe Mountain God x Hyung? Hehehehehehe (▰˘◡˘▰)