Tamjung Hong Gil-dong: Sarajin Ma-eul (2016) online

Private detective HONG Gil-dong is seeking revenge for his mother who was murdered in front of him when he was a child. An investigation determines that the assailant is a man named Kim, but when he arrives at the Kim's house, he finds only the man's grandchildren who tell him that their grandfather was kidnapped. Hong tracks down the vicious kidnapper with help from the grandchildren, but senses that he is not acting alone. The mastermind is the notorious leader of a massive cult - a man with a terrifying, bloody secret in his hands. Now, Hong decides to face him once and for all.