To wake E.N.D,Natsu has to open his book. That's all.
The thing is,we don't know Zeref's recent past, the last 82 years or so.. What he was doing,how he was doing,how long he evaded people. People are only assuming too much. All we know is his start of darkness and his time with Mavis. It's pretty clear from this chapter that there is more to his past than that.
For instance,we don't know how long he was in sirious island. We don't know what lead to him valuing life so much or why he created that empire. Plus,if you remember fairy tail zero,Zeref entered a city while passing by Mavis. Plus,while he trained Mavis and company,it didn't act up.
Some people think that this reveal is cheap because it contradicts with Zeref's character. But well,not everyone can be smart,unfortunately. They haven't noticed that his character is a pile of contradictions. So it fits. Plus,I wouldn't forgive Mashima if this arc served only to cockblock our Zeref arc.