Sword Art Online 19 (light novel): Moon Cradle — Reki Kawahara

Read the novel that ignited the phenomenon!
After three hundred years of conflict, a new era begins for the Underworld. The Lost Child of Vecta, who seemed to appear out of nowhere, has defeated the Dark God and given the world its first taste of peace. The human realm and dark realm have begun to intermingle, sparking the beginning of a much-needed cultural exchange. But while some see this as an opportunity to bury the hatchet and move forward, others are determined to sow the seeds of chaos anew. High in Central Cathedral, Apprentice Integrity Knight Ronie Arabel's heart sinks as she receives foreboding words from Swordsman Delegate Kirito: "At this rate, there's going to be another war."