Sword Art Online Novel Volume 18 is written by Reki Kawahara.

The second day of the Final Stress Test rages on. Having defeated Sinon and Leafa--even with the added might of their godly super-accounts--Gabriel drives Alice into a corner. Meanwhile, in another corner of the battlefield, Asuna is pushed to her limits as she defends Kirito from the army of Darkness. But just when all seems lost, a voice cries out in Kirito's heart, urging him back into the world. Grabbing hold of that light in the darkness, the black swordsman awakens...