Sword Art Online 17 (light novel): Alicization Awakening (Volume 17)

Read the novel that ignited the phenomenon!
The final stress test bleeds into its second day. Driven to desperation by the Integrity Knights' coordinated assault, the Dark Army reveals the ace up their sleeve! Critter, who has been observing the battle from the Ocean Turtle, has successfully created a gateway into the Underworld and masked it as an invitation to "an open beta test for a new, unannounced VRMMO." This digital invite bolsters the ranks of the Dark Army by the tens of thousands! Having already been pushed to her limits, the thought of taking on a fresh wave of bloodthirsty enemies fills Asuna with dread. But just when it seems like all is lost, two new deities of the Underworld appear to take up the fight: the Sun Goddess, Solus, and Terraria, the Earth Goddess. And the ones in possession of these super-accounts are none other than Sinon and Leafa!