Thread:B214/@comment-30924344-20170604084454

I believe this story takes place after Volume 12.


 * Rias mentions V12 and is very distraught about remembering it so it makes sense this would be the case, if it's right after V12.
 * Rias mentions how Issei would always pay attention to Ophis and Koneko in the living room during weekends. Ophis moved into Issei's home at the end of V12.
 * Issei is calling Rias by her first name.
 * The story is about a person wanting to avenge the Hero Faction who were all defeated in V12.

All of this sounds like it happened recently after V12. 