User blog comment:B214/Volume 21 info & spoilers/@comment-78.95.78.251-20160328114810/@comment-27876048-20160328141858

@anon while that would be cool, the world will be destroyed in the process

@Mkprovince Issei's DxD power-up is a combination of Ophis's power and Great Red's flesh and when he had that Scail Mail Gigantis form in volume 12 that was because Great Red let Issei borrow some his power