so I have this theory and it came to me when someone made a comment of a photo of natsu and Lucy holding END. I suddenly remembered that Lucy is a writer. And I thought about how natsu's life was staked on a Book which not only linked his life to zeref, but also makes him want to kill him. right now that book is in zerefs hands. But I think that by the end Lucy will get that book and rewrite that spell and save natsu.