User blog comment:Kiraofthesky/END/@comment-27019518-20160408204829

When Igneel reveals himself during the Tartaros Arc he says "I'm confident you can defeat END." This could be hinting that Natsu and END are split personalities so not only does the book hold his Etherious form and powers it also holds his darker self. This could also explain why he gets hurt when Zeref blasted the book because it has a part of his soul and or it could be the book is what is allowing Natsu's soul to stay on Earthland because Natsu was originally dead and with that gone he will die