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, the opening night of "The Pilgrimage of Heinrich," an opera starring acclaimed actor Chikaomi Tsurugi and the popular idol Ayaha Oribe, was subject to a mysterious tragedy. Almost everyone present in the opera house, including the cast, crew and audience, as well as Chikaomi's son, Yashiro Tsurugi, 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.
In Chapter 5, it is revealed that the Mass Disappearance was caused by Gharnef, who possessed Chikaomi Tsurugi and transported everyone to the Idolasphere in order to perform the Opera of Shadows and resurrect the Shadow Dragon Medeus. It is also revealed that the reason Tsubasa was spared was because Tiki saved her.