User blog comment:FairyRaven/Fairy Tail Chapter 401: Igneel vs. Acnologia, Review and Commentary/@comment-11721917-20140922125631

While it did made me question where the hell are the other dragons (especially Metallicana), the chapter is good enough. That roar though. Makes Natsu's look like a spark.

Maybe Gray will appear later and we'll have a two-on-one battle?

Also, opening the book might summon/revive/whatever E.N.D. Just my wild prediction.