User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-1359322-20130610122235/@comment-86.121.44.16-20130711193846

My opinion is that Zeref knows of some kind of prophecy where he can finally rest from his deadly curse only with the help of Natsu. And that is only when Natsu is strong enough to kill him, like zeref pointed out that natsu is still to weak to kill him in the tenrou arc.