User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-118.136.106.212-20150523130151

E.N.D is non other than Etheria Natsu Dragneel, the younger brother of Zeref. Zeref created Natsu to kill himself. That's why Igneel was trying so hard to stop the revival of E.N.D, which I think will kill or affected Natsu.