I'm replaying Stormblood on an alt, and decided to do the Four Lords storyline again because I greatly enjoy it.
I'm not going to go into spoilers, but basically Koryu had the power to destroy the world. If it wasn't for some last second heroics on the part of us and our allies, he would have done just that.
But it seems like Koryu could have caused a Calamity. In general, there seems to be some serious untapped power in the Far East - from the relics, to the kami, to the Four Lords themselves - that could provide more than enough power and aether for the Ascians' purposes.
I'm curious as to why they never seemed to use any of it. They are almost exclusively focused on Eorzea and its best tribes.