Maybe this is obvious to others but after thinking on what we’ve seen of Myths I have a theory. Given what we know of primals, it strikes me as odd that we have no record of someone summoning a primal since the Third Astral Era. The Ascians were more than willing to share the secrets of summoning, and there’s no reason to believe that religious belief had waned in that time, especially since worship of the Twelve dates back to then.

Silvertear meanwhile, is the place where all Aetherial currents meet and while this is purely speculation, I wonder if it’s not the place that primals return to.

So now on to the actual theory. What if the presence is the collection of millennia of worship, sealed by Hydaelyn beneath Silvertear to prevent primals from destabilizing Etheirys/becoming to powerful? Looking at it from Her point of view, primals drain aether from the land and are the perfect tools for causing calamities. That, plus the fact that the Sundering was partially about preventing a powerful deity from using its strength to solve its followers ills, would mean She has good reason to stop anymore summoning. However, she can’t and shouldn’t force people to stop worshipping, nor can prevent the Ascians from teaching the mortals how to give their gods life. Thus, she seals them, much like the Allagans sealed Bahamut and the Warring Triad (though without the fucked up enslaving part.) It would explain why she has Midgard defend the seal, as well as the scene with the primals in the 1.0 trailer where they seem to be “released.” Gaius’ rage at what he thought to be false gods would also have additional nuance if what stopped his invasion is a literal jamboree of primals.

Guess we’ll find out soon