User blog comment:Bluefox87/Chapter 416 Natsu is E.N.D...WHAT?!?/@comment-5303512-20150310045625

I think that Natsu is just the Physical Form of E.N.D and we all know that the Book of E.N.D is E.N.D's soul. So maybe 400 years ago, E.N.D's body was destroyed and his soul was sealed back into book form. And so he would communicate through Mard Geer giving him orders as the Master of Tartarus. That would explain him talking with Mard sometimes and why Zeref haven't destroyed to book yet. As for Natsu being E.N.D's body, maybe without E.N.D's spirit, his body had his own personality, and if he opened the book his soul will return to Natsu. (Just like in Heroes where Matt Parkmen sealed Sylar's consciousness inside him while Sylar's body took the personality of Nathen Patrelli)