Some supernatural thingy called Bulgasal has been affecting Korea for 1,000 years. Its identity and the genesis of Bulgasal is shown at the very end after the series goes for a reiteration of failed attempts involving the three leading roles to sort out what's going on. It's boring beyond belief, but at the time was the only new Korean stuff on netflix (Bulgasal enticed me to check out shows from other countries, and I found decent movies/series from the Hispanophone world, e.g. Toc toc, Tell me when, Outlaws, So much love to give, ...).
A lot of the stuff in Bulgasal actually makes no sense.
The writer and producer (and whoever approved and financed the project) should have made a 4-episode mini-series, paying close attention to stuff making sense, and highlighting the scenes in ancient Korea (the costumes are well made and very different from the standard Joseon stuff).