The revelation of Gray about the true purpose of the Book of Zeref caused a fight between him and Natsu in the "Fairy Tail" chapter 504.

The chapter saw Natsu facing Gray, who persistently asked him if he was E.N.D.

Natsu's transformation into a huge and powerful creature came just when Dimaria was about to torture him and Lucy. Dimaria thought that Lucy should pay for making Brandish go rogue. However, just when Dimaria was about to poke Lucy's eyes, things went dark and she saw Natsu transform into his Dragneel form. There are speculations that in that very moment, Natsu's E.N.D. transformation was already complete. In the manga, E.N.D. means Etherious Natsu Dragneel.

The battle between Natsu and Gray in Chapter 504 seemed to indicate that a reconciliation might be out of the question for the two former friends. While Natsu told Gray to get out of his way, Gray did not, and instead told him that he wanted Natsu to answer truthfully if he was E.N.D. Gray revealed that the demons of the Book of Zeref were tools created by Zeref himself to end his own life. He also told Natsu that one of the prized demons of Natsu was the Etherous Natsu Dragneel.

Gray told Natsu that he would kill him before Natsu even kills him. It remains to be seen how long the two will fight, since Natsu said no one could stop him from fighting Gray. However, there are speculations that he will stop once he notices that Lucy is already immobile.

Meanwhile, the release date of "Fairy Tail" chapater 505 remains unknown.