Does anyone have a theory on what the monster in the 둥근 천장, 금고 is? The monster is as yet unknown, has been alive for thousands of years, and can eat away at your soul. That could be anything...

Gog Magog, Demons that of an “evil and destructive nature” and “caused great corruption on earth. They are trapped there until doomsday, and their escape will be a sign of the end.
the Rakshasa hold a wide range of powers, including shape-changing, creating illusions, and working powerful magic. They tend to have toxic fingernails 또는 claws and they eat people, to boot.
Wendigos are the embodiments of gluttony, greed, and excess, are violent and obsessed with eating human flesh. They are never satisfied with killing and consuming one person.
Encantados are shapshifters, often dseen as seas snakes, that use various mind control techniques and can inflict illness, insanity, and even death.