Whizbang Dustyboots
Gnometown Hero
Two cultures at war with one another, who are deeply suspicious about the entire way of life of the other side would be an incredibly ripe environment for the Cult of Tharizdun to infiltrate and each side could easily be fooled into thinking that it was the other guys who were in bed with the cultists.I couldn't see a connection when I glanced over the wiki about this setting. It is an island divided up between the Knorr barbarians and Charonti necromancers. Can you please tell me a little more about the setting and how it would be suitable?
Fast forward to after the apocalypse, you have two detailed settings to use, but neither of them enormous -- you're not trying to figure out how to convert whole continents to cover the new status quo, but two island cultures, one of which -- from the outside -- already comes off pretty post-apocalyptic to begin with.
I'm not 100% clear on what your vision for the campaign is, but a small focused pair of settings with an in-built NPC motivation -- "it was those other guys who did this to us!" -- would get you pretty far down the road, in my mind.