Alright, thx for notifying me. Tbh i can't really think of a system for that except maybe as a joke. Have the same system for awareness of the darkness and copy it over for the environment and global temperature drop. Ex: i drop the global temp by a bit, this causes the southern cities to be more habitable, hence their lords there have no motive to do anything, but the northern lords have their cities damaged, which causes them to think about how to reverse it. Now, HOW they reverse can be magic BS, just burning a bunch of things, or something else i can't think of. This way the system encourages using the weather system only when the war is so big that the lords dont have a chance to really fight it.
Other ideas i can think of are:
Have the sun death power be permanent but also a really complicated ritual that needs a dark empire and the other power be temporary one-off spell.
The provincial hierarchy you mentioned is really good and can be tied into casus belli. Ex: The duke wants to war on the nearby empire because they control one of his duchy's towns.
And that's about it, thanks again for the info. Can't wait to see how you develop it further *girly squeal*!