User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-172.56.3.122-20141216004353/@comment-115.186.98.152-20141217150057

Maybe E.N.D Is A Demonic Spirit, that is made to combine with Natsu, but is too strong for Natsu to Control and will try to kill Zeref in that Evil form and eventually try destroy the world.