User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-4933386-20150111043522

I have a new theory of what END could be. I think that it is a dragon that once entered Natsu's body and possessed him which is why Igneel tried to destroy it. But the only thing Igneel could do was eject that soul/whatever out of Natsu body and seal it back into the book.

Which is why Igneel enter Natsu body to prevent END from going into his body again. Which might explain why Zeref knows Natsu because they somehow met while the dragon was inside him.

Now that Igneel is out of the body; END could potentially enter his body again if the book is opened once more.