N - Manga

My vision and my heart were completely taken over by "N." The case of five missing junior high school boys, the massacre of an entire elementary school class. A car crash resulting in the deaths of two people. The letter "N" was left at each of these crime scenes. An urban legend circulates, suggesting the main culprit is 'N,' a religious group thought to have collapsed 2,000 years ago. What exactly is "N"?