The Mass Disappearance is the inciting incident in the plot of Tokyo Mirage Sessions ♯FE from which the rest of the story follows.
Five years prior to the game's present day, an opera starring acclaimed actor Chikaomi Tsurugi and the popular idol Ayaha Oribe was subject to a mysterious tragedy. Most everyone present in the opera house, including the cast, crew, and audience, vanished in clouds of black mist. The only person present to be spared was Ayaha's younger sister Tsubasa Oribe, who was seated in the audience. According to a newspaper that appears at the end of the opera cutscene, It is estimated that around a thousand people disappeared in the event.