User blog comment:Omega natsu2/Chapter 357: The Nine Demon Gates, Predictions./@comment-24066070-20131018172211

Seems like someone already mentioned "Zeref" would be the GM of Tartaros. I think this is very possible, since it seems impossible for someone to live 407 years. Zeref probably made a demon into a "copy" of himself so he could live longer (perhaps forever). It would also explain his obsession with Natsu (since Igneel couldn't stop him, he hopes Igneel's son can). Also "Zeref" hasn't done anything for so many years, then recently he declares war on humanity and Tartaros starts acting almost right away. Perhaps the "keys" gathered by Hades were needed to summon E.N.D. but he was already existing.

Further predictions:

The Ten Wizard Saints will become the acting Council, the Gods of Ishval may make an appearance.

Tempestra will attack Fairy Tail, this would explain the Lucy's frightened face.

Kain Hikaru and Rusy Rose will be members of Tartaros now.