User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-5303512-20140314233709/@comment-5598380-20140315000818

It's not determined if Zeref is already on Tenroujima when Gildarts met Acnologia at the sacred peak of Mt. Zonia. But yeah, Zeref's anger may not be the only reason why Acnologia appears.

About the Century Quest Gildarts failed to complete, it's not yet revealed what it is. It's still a mystery.