User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-14.201.211.143-20150720115807

From what I read Diabolus Dragon is only from Ophis giving Issei power it didn't seem to have a thing to do with Great Red, because of this I think to beat 666 a combo of the Great Red form from vol12 and the new D∞D form will be the form used, or everyone fires everything at it