I Got the Genre Wrong for 20 Years is a Korean web novel that follows the story of a protagonist who transmigrated two decades ago. Initially thinking he was in a hero story, he defeated the Demon King, only to be tasked with saving the world from a villainess. Filled with action, fantasy, and harem elements, the novel explores themes of betrayal, magic, revenge, and complex family relationships, making it a captivating read for fans of dark fantasy with strong character development and intricate plot twists.