UNDEMON is a 2D platformer set in an area filled with paranormal activity. The main character is the virgin ghost of the Moon, who goes through various stages associated with his life. Each location evokes in the ghost a sense of test that he must pass at any cost. The central character is based on popular heroes in Korea.

Controllers and gamepads are supported, as well as keyboard and mouse. One gloomy day, powerful lightning struck a dilapidated tomb, from which the ghost of a woman suddenly emerged. However, she does not remember anything, but suffers due to a lack of understanding of what she is required to do. Someone finds a virgin ghost not far from his house, an otherworldly creature notices this and enters the house.

The gamer will have to explore a vast two-dimensional world, interact with the environment and solve logical problems. The main mission is to understand why the central character became a ghost, and whether there is a way to return his former body. As you progress, more information and memories are revealed that help you understand who did this to the protagonist. The game locations have a variety of secret places and secondary areas to explore. Along the way you will come across bosses and bonuses that help in battles and provide additional abilities.