User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-71.222.52.171-20151209065359/@comment-7418318-20151209155158

With E.N.D. being called Zeref's strongest creation even by Zeref himself I too think that there's more in that book than just the truth.

I mean come on, sure Natsu is strong and by now has surpassed Zeref's other creations, but that's only due to hard training and experience. If he had been pitted against a Kyoukimon before having his second origin unlocked, he'd have been next to helpless.

And even with second origin he had to work together with a turned-Devil-Slayer Gray to overcome Mard Geer. How is THAT Zeref's strongest creation? And they were always talking about strength itself, not the "potential to one day become the strongest, if he put enough effort into it."

That leads me to believe that locked away within the book there's the demonic or curse-related side of his powers.