The Immortal Dusk refers to an incident that took place during 2020, where the actions of Geist using premade plans from ex engineer Jyotarou Amagi and secretly Mama cause a fourth network crisis by corrupting the goddess of The World, Aura, disrupting the Akashic Archives and forcefully Real Digitalize Users of The World, as well as various owners digital technology into the game.
Spoiler warning: Plot and/or ending details follow.