User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-7418318-20140705180252

I've just reached the end of the Edolas arc while re-reading the manga and it made me wonder something.

Could the reason why Tartaros has problems with summoning E.N.D. be the additional magic that Mystogan cleansed from Edolas and pumped into Earthland?

I mean why would Zeref create a Demon that can only be summoned after the enemy is pretty much already defeated? It makes no sense, unless this matter was unforeseen. But what could such an unforseen matter of a big enough magnitude to block E.N.D.'s ressurection be? Maybe the magic of a whole world that allowed several large islands to float in the sky?