In the early Joseon Dynasty, strange rumors began to circulate among the people.

The rumors said that once the sun set and darkness fell, a mysterious entity—neither human nor beast—would appear somewhere in the mountains and fields to harm people.

No one knew where it came from or for what purpose it harmed humans.

People called these mysterious beings 'Gwimul'.

And soon, a single warning began to spread among the commoners.

"Never go up the mountain after the sun sets."