User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-24784846-20150119163520/@comment-9595671-20150119180519

We don't actually know due to how Mard Geer was killed, that seems to be important somehow as END is supposedly still sealed. *scratches chin*

Its mysterious, especially since Igneel couldn't beat END, yet END happens to be his foster son.