Watch Monstrum for Free: How to Stream the Movie Online at 123Movies. . Ancient Korea, 1506. The tyrannical King Yeonsan-gun of Joseon is overthrown by his half-brother Jung-jong, whose reign begins with a blood bath. Over the years, traitors plot against him, sinking the kingdom into chaos. In 1528, frightened rumors come to royal palace, regarding a mysterious creature, known as Monstrum by the peasants.