User blog comment:Miskos3/Fairy Tail Chapter 506: The Broken Bond, Review/@comment-5303512-20161017131353/@comment-5303512-20161017145959

Wow, you are just willingly forgetting that the demon marks corrupt Gray or are you just pulling my leg? Besides that, they have always been books of Zeref, while Natsu's book was separate from him, I don't know what the history is if Those two have always been separate or they were force to be separate, but from what was shown, the book definitely had some effect on Natsu to have turned him into his END self.