User blog comment:IamJakuhoRaikoben/Chapter 389: The Twin Dragons vs. The King of Hades, Review./@comment-24467372-20140620101344/@comment-7418318-20140620101828

So Zeref might have infiltrated one of his Human-looking Demons into the Dragon's training programm to gain that power for himself? Interesting theory.

But if it turns out to be the case, I hope that Acnologia was able to escape Zeref's control (either because of gaining the DS magic or due to the transformation into a Dragon), rather than still being his puppet.