User blog comment:FairyRaven/Fairy Tail Chapter 372: Breach, Review and Commentary/@comment-7418318-20140221101604/@comment-7418318-20140221103325

The scenario that popped into my head is as follows:

Zeref tells Natsu that he's a Demon from the Book of Zeref (BoZ) and later (or right from him) Natsu learns that the only way to kill Demons from the BoZ is to destroy the book itself. If that's not done they'll come again and again until they'll eventually win.

He tells the other FT members they have to destroy the Book, but keeps it secret that this will also kill him, since he's also from that book.

After the book is destroyed he's starting to disappear and we get a heart-wrenching scene between him and Lucy.