It was stated a few times, that Norse, under the wish of Yaggdrissil, is trying to get aqua. Precise details haven't been delved into, but it's implied that Yaggdrissil wants revenge on Aqua or wants to absorb Aquas's soul
Shinto are kinda useless if another pantheon can invade them so easily even in a territory they loaned an outside god coming in without permission is still an act of aggression worse if they are three
You know, I hope this isn't a kidnapping or family injured trope. I have read enough about his past legends. I need him to enact one now. To just Butcher everyone in on this raid.