"As long as people don't forget, recreating it will be possible."
If there's any valuable lesson to be picked up from this arc, it's that coexistence between humans and demons of this world are possible based on this era, and it's something the two sides may want to consider following when the war ends.
"underworld dragon ran away during the previous invasion"
Wow, symbol of power indeed. It's just a symbol and not an embodiment of power.