Sword Art Online Novel Volume 21 is written by Reki Kawahara.

A year has passed since Kirito and Asuna’s return from the Underworld. With them is Alice, who has been given a physical body. But their peace is quickly broken when the three are suddenly thrust into an unfamiliar game, Unital Ring. This new world is a survival MMO that fuses together every VRMMO created with the Seed program. As soon as it starts, Kirito finds himself with nothing but his underwear. Now, he’s got to challenge a mysterious VRMMOSVG under desperate circumstances!