Everything can be covered in shadow.  This is the significance of C.S. 13.  Two players appear beneath the Ikaruga no Kagami with no real recent memories.  Ikaruga uses that as a chance to move forward, “What should I do?”
An original novel derived from .hack//G.U.  Pushed forth by original characters, it takes place between volumes 2 and 3.  Warning, while the main story remains the same, AIDA’s setting, the twilight epitaphs, Harold, and others have been tweaked.