Nergal Black is a mysterious and enigmatic character from the video game 'Catherine.' He appears as one of the patrons at the Stray Sheep bar, where the protagonist, Vincent, often spends his nights. Nergal's role in the story is subtle, acting as an observer and sometimes providing cryptic advice about life and relationships. His presence adds to the surreal and philosophical atmosphere of the game, making players question reality and morality in the choices they make throughout the narrative.