User blog comment:IamJakuhoRaikoben/Chapter 451: Fairy Heart, Review./@comment-4829046-20150907165957/@comment-4829046-20150908170326

Yeah, but they ARE Zeref's forces and it seems he doesn't plan to use a Demon army so if the Spriggans are beaten in this arc(which will happen for obvious reasons) then Zeref and Acnologia would be alone. Sure, that would make sense due to their power, but let's face it, that would just be a Monster Boss battle out of an RPG and not a proper war. It makes more sense to tie it into this arc since A)Alvarez was created in order to destroy Acnologia, B)Fairy Heart is the best way to deal with the two and C)if our troll-curse God appears, then Fairy Heart can deal with him too :P

The big problem with having stupid arcs in-between is the lack of a legit threat. We moved onto a different continent because all the Dark Guilds on the mainland are dealt with and even the Avatar cult was wiped out in an instant. It would make absolutely no sense for an even greater army to appear after Alvarez. That's called ass-pull development. Zeref was the key player behind all developments in FT and for once, he is actually being an antagonist instead of an observer so it is pretty much the prelude to the 'FINAL BATTLE'. And the only reason to prolong it would be to use another time-skip card, which we know isn't going to happen.