do you think that Zeref knows that Natsu has already transformed into end?
I’m not sure. He might have been able to sense the change in Natsu’s magic and he has the book (I don’t know if that would change with Natsu's transformation or use of curse power). What Zeref wouldn’t know is what triggered the change. It could be that he could have seen Natsu and Grays fight and assumed Gray triggered it? I don’t think this will be as plain sailing as Natsu hopes, no matter what Natsu is tied to the book. Whatever the outcome may be, there is certainly going to be some interesting revelations.
I don’t want Zeref to be defeated this arc, he is supposed to have godly levels of power. This war was about him messing with Natsu. Think about it, if Zeref’s main goal was truly world destruction, then surely he would have done it years ago when FT was still under the tenroujima protection spell. Whatever his plan is that he told Gray, a part of that plan involves Natsu using his demon powers. It’s the only reason why Zeref let Happy take Natsu away, why he’s obsessed with trying to break Natsu, he wants him at full power. He knows Natsu’s emotional rage is what makes him access his dormant powers.
This is why I think Lucy is in danger. Although Natsu knows she can handle herself, he doesn’t view her in the same way he see’s Gray and Erza. If any harm comes to Lucy, it flips that switch in him very quickly. Lucy is the only one he doesn’t challenge to fights constantly (other than wendy), it’s because he hates to see her harmed in any way.
Zeref’s a smart guy and he’ll figure this out quickly. Natsu’s ‘nakama pissed’ right now, but ‘lucy’s hurt’ pissed is another level. No one hurts waifu is the top level of pissed in FT in my opinion, compare the faces in those scenes with other angry scenes. It’s just not the same lol