User blog comment:BlackWizardZeref/Fairy Tail: Chapter 380 Hell's Core/@comment-7418318-20140411142701/@comment-7418318-20140411143532

Or he could be a sealed up form of E.N.D. and Kyouka is going to fully unleash him.

Though that'd be so cliché that I'd much prefer the "book-bearer" or guild-ace theories.