User blog comment:Miskos3/Fairy Tail Chapter 506: The Broken Bond, Review/@comment-5303512-20161017131353/@comment-1359322-20161017140746

@CNBA3, Excuse me, you never said Natsu and E.N.D. are separate entities. I'm too lazy to search too detail so i'll give three that i remember clearly.

"Or it could be that there is a separate entity in the book but they are bound together, like that movie where the dragon gave that body a part of his heart and their fates are tied together."

- My review blog on Chapter 499

"Or perhaps if say Natsu dies and that is when E.N.D. Awakens."

- Rai's review blog on Chapter 497

"Perhaps 400 years ago, E.N.D. was sentient and developed a grudge against Zeref and refused to kill him like he wanted just to let him suffer."

- Rai's review blog on Chapter 497