Midnight is trolling.
From what I can tell, though, there hasn't been any specification given yet.
–-
Oh, and something I totally overlooked earlier.
- Brain said, when all six members of Oracion Seis are defeated, "He" will be revived/released/whatever.
Any guesses on what this might mean? Somehow I'm doubting it's going to be Zeref. That seems too simple, and I thought the Balam Alliance wanted to do that. One of Zeref's demons, like Lullaby or Deliora, perhaps? It's been a long time since we saw one of those.
Maybe this arc's true final boss will be released? The Real, Inner, Dark Brain? Gerard?
I don't think we'll have to wait too long, but I'm placing my bets on "Yami Brain."