User blog comment:LegerPrime/Natsu is E.N.D./@comment-24403746-20150915125202/@comment-2214433-20150915132953

From what I understood, E.N.D.'s soul was sealed into the book that Mard Geer was carrying, and they needed Face to break the seal and amass enough curse power. At the very least, Natsu himself shouldn't randomly turn into E.N.D. because his demon soul is elsewhere, it's the friggin' book. So yea, this theory is ... quite random to say the least.