User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-203.215.121.152-20140913114834/@comment-76.124.155.27-20140913144853

I like the theory that the dragons sent the dragon slayers forward in time. Most likely a response to Acnologia finding that dragon cave under the Fiori castle. If you remember there were estimated to be a hundred dragon corpses there. And we can assume that the last 5 dragons took their children and ran to hide them till they were strong enough. I don't think Lucy per'se has a connection to Zeref but celestial wizards in general do since they use magic in it's purest form and can most likely understand what the one magic is better than most.

I got the impression that the demons thought Acnologia would stay neutral but since they obviously fear him as much as everyone else I'm gonna guess that his overcharged dragon slayer power is enough to destroy them while they dont have the means or the power to kill him.