Sword Art Online Novel Volume 28 is written by Reki Kawahara.

As the battle between the Underworld Space Force and Central Cathedral intensifies, a long-awaited guardian of the Underworld awakens at last. Second-ranked Integrity Knight Fanatio Synthesis Two returns to the front lines! And in the real world, Asuna is in for a surprise of her own, two years in the making… But for every tearful reunion, there awaits a dangerous new encounter! Kirito and the gang can't afford to let their guard down for even a moment!