User blog comment:GotMilk5101520/How will the Tartarus Arc Part 4 will end?/@comment-5303512-20140709034950/@comment-5303512-20140709050409

I thought it was made clear by Zeref it was a chose Igneel made, and that Atlas just said Igneel failed to kill E.N.D. I also don't think E.N.D would return without being strong or something like that, maybe even enough for him to beat Igneel and other dragon's of his caliber. That's usual how all these revenge villain plots play out.

Or maybe Igneel and the other Dragons that trained Dragon Slayers became weak by giving a portion of their powers to the Dragon Slayers. That would explain why we see images of Igneel when Natsu is "all fired up".