The Dark One, also known as Valentine Morgenstern, is a central antagonist in the Shadowhunters series, which is based on the books by Cassandra Clare. He is a powerful and ruthless leader of the Circle, a group that seeks to eliminate all Downworlders, including werewolves, vampires, and warlocks. His actions are driven by a desire to create a world where only Shadowhunters exist. Valentine is characterized by his extreme beliefs and willingness to use violence to achieve his goals. He possesses significant knowledge of demon summoning and has access to ancient artifacts, making him a formidable foe for the protagonists, including Clary Fray and Jace Wayland. His complex motivations and tragic backstory add depth to his character throughout the series.