Monstrum (2018) on Netflix

Plot summary

Joseon, in the reign of King Jungjong. Plague spreads through the country and the kingdom slides into chaos, while frightened rumors reach the royal palace of a mysterious creature the peasants call Monstrum, said to be roaming the country. King Jungjong takes the throne from his half-brother, and his reign begins with a bloodbath. Traitors plot against him over the years. Yoon Gyeom, a loyal subject of the king, is caught between the creature that threatens the king's life and the group of people working to depose him.