User blog comment:IamJakuhoRaikoben/Chapter 451: Fairy Heart, Review./@comment-24963954-20150907103559/@comment-24264679-20150907145730

Don't magic need to disappear for END seal to broken? Fairy Heart poses a great threat to that plan. Also it was implied Acnologia fears END which was created by Zeref so I think it is safe to assume Zeref is stronger then Acnologia.