User blog comment:FairyRaven/Fairy Tail Chapter 401: Igneel vs. Acnologia, Review and Commentary/@comment-25229960-20140924230023

Guess: Gray will open END book since his father said that demon powers can only kill END.

Also, I think it'll be Natsu solo since Crunchyroll's translation of the next chapter was "Fire Dragon's (not steel) Iron Fist" so I don't think Gajeel will join in.