The Battanian kingdom is based within a giant caldera of a super volcano.
A super volcano, that due to tectonic shifting, is now creating a volcanic mountain next to Rhotae. Why else would there be a giant solo mountain in the middle of plains?
This obviously erupts between Bannerlord and Warband. a massive blast that levels Battania, and western empire entirely. wiping out cities across the entire continent, and flattening entire mountain ranges. Changing the course of rivers.
In the end it was not iron that ends the Empire, its fire.
That would be an interesting idea. It would certainly explain why the Western Empire, once verdant, looks bleak and barren in Warband.