The 2.0 Teaser trailer and 1.23a patch quest. [Speculation and Spoilers]
So...I think we can reasonably infer through the trailer and the dialogue of the latest GC mission that Louisoix is summoning the Twelve in the teaser. He isn't protecting us per say, he's protecting the summoning grounds.
Through in-game dialogue, he states that to take the appropriate summoning grounds, we will need to bring the full strength of the Eorzean Alliance to bear against the VIIth legion.
A massive clash between the Alliance and Empire....sounds like the battle in the teaser trailer, eh?
And the achievement for the Dalamud Horn says that it's the reward for "Completing the ENTIRE scenario centered around Dalamud and the Empire's Meteor project."
So here we are at last...
Louisoix is summoning the Twelve, and the giant barrier in the teaser trailer is to protect those grounds.
Mystical Twelve-fold Kamehameha incoming to stop Dalamud from making contact? Speculations begin!