Boss Monster is a card game where players take on the role of video game villain bosses. The objective of the game is to attract and defeat as many heroes as possible. Each player builds their own dungeon with room cards, which are used to lure in heroes. The heroes are then defeated by playing trap and monster cards. The game ends when a player has collected ten souls or when all heroes have been defeated. Each player's dungeon is scored based on the number of souls collected, with the player with the highest score being declared the winner.