Disclaimer: I do not own DXD but my book does. Rias and her peerage was given a book by a mysterious person. Two mysterious men to be exact. So they invited Sona and her peerage, and for 'some' reason Raynare, Kalawarner, Mittlet, Viser, Ajuka, Ravel and Riser's former peerage, Katerea, Serafall, Sirzechs, Mavis, Grayfia and their human friends.