User blog comment:Derax-Pagani/Fairy Tail Chapter 493 White Dragneel Review/@comment-4933386-20160711124707/@comment-1359322-20160712170815

IDK. Natsu could have been revived as E.N.D. 400 years ago and created the Curse as well as doing other acts. Igneel eventually fought Natsu and successfully split E.N.D. into two entities the book and the boy he was based on. That would also explain why Igneel didn't want Natsu to open the book.