Sword Art Online, Vol. 19 (light novel): Moon Cradle

After three-hundred years of turmoil, a new era begins for the Underworld. A lost child of Vecta, who seemed to appear out of nowhere, has defeated the Dark God and cemented peace for all the realm. But at the hear of human lands, atop Central Cathedral, Ronie Arabel, newly promoted to Integrity Knight apprentice, receives foreboding words from Kirito. "War will come again."