User blog comment:IamJakuhoRaikoben/Chapter 242: Acnologia, Review./@comment-111.68.59.76-20110709082935/@comment-111.68.59.73-20110709083645

Also the question none of us know; How does Zeref know Natsu? But I have a theory on this; since they say Zeref is the root of magic, maybe he knows lost magics like Dragon Slayer magic and by this he knows that Natsu is the only person who can kill him.