Sword Art Online Novel Volume 24 is written by Reki Kawahara.

A reunion with Seijirou leads Kirito, Asuna, and Alice back to a version of the Underworld where 200 years have passed since the Great War. There, they find not only the descendants of Ronie and Tiese, but also a man claiming to be the leader of the Integrity Knights, whose eyes resemble his eyes-the one Kirito lost. Meanwhile, in the world of Unital Ring, the ultimate malice targeting Kirito and his friends finally reveals itself!