Sword Art Online 18 (light novel): Alicization Lasting (Volume 18)

Read the novel that ignited the phenomenon!
The second day of the final stress test rages on. Lisbeth, Silica, and Asuna push through the pain as they hold out against the Dark Army. But their efforts fall short as Kirito is captured by PoH, the former leader of Laughing Coffin! With PoH's twisted heart set on subjecting Kirito to the culmination of several years' vengeance, he prepares to end him once and for all. But when hope seems lost, a voice cries out in Kirito's heart. The voice of the person he lived with, laughed with, and fought with for years. The voice of his best friend...Grabbing hold of that light in the darkness, the Black Swordsman awakens...