User blog comment:FairyRaven/Fairy Tail Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Review and Commentary/@comment-5303512-20140214102756/@comment-1359322-20140214162241

If E.N.D. is just a book, then why would Igneel fail to destroy it. Also Atlas Flame said that E.N.D. is the strongest Demon from the Book of Zeref. I doubt E.N.D. is just a book that spawns Demons.