User blog comment:LordLuminous/Fairy Tail Chapter 411 Theories and Predictions/@comment-7418318-20141123173547/@comment-7418318-20141124101405

The two Dragons forming a temporary alliance... or rather a cease-fire to deal with Face or END would make sense from the title, but so far Acnologia seemed a bit too obssessed with killing every Dragon to take note of anything else.

But if we take the literal meaning "something for something" (I think), it might hint at a sacrifice to fuel whatever Lumen Histoire is going to do. Maybe Makarov? Or Elfman? Mashima must have had something in mind for him when he made him stay behind, though I can't really see what he can accomplish against Mard or Face that wouldn't make everyone scream bullshit off the top of their lungs.