The New York Times has introduced a documentary audio series that follows Rukmini Callimachi, who covers terrorism for The Times, on her quest to understand ISIS. Today, as a special episode of “The Daily,” we offer Chapter 7 of “Caliphate,” in which Rukmini examines what ISIS left behind as their hold on Mosul crumbled. For more information about the series, visit nytimes.com/caliphate. This episode includes disturbing language and scenes of graphic violence.