The Immortal Dusk refers to an incident that took place during 2020, where the actions of Geist, who was using premade plans from ex engineer Jyotarou Amagi and secretly Mama, cause a fourth network crisis through the corruption the goddess of The World, Aura, that was introduced via the use of a virus attached to a Doubleware as well as the disruption of the Akashic Records and forcefully Real Digitalize users of The World, as well as various owners of different digital technology throughout the world.
Spoiler warning: Plot and/or ending details follow.