The current theory behind Kuriotate OOS has to do with the third ring, but the odd thing is that
A) even a human Kuriotate player can cause it
and
B) it (the OOS) wasn't introduced until MNAI 2.3
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The Hyborem OOS (which is still ongoing in MNAI at least) is a holdover from a late FFH2 patch, possibly g?
Anyways, before, lets say patch 'g', if Hyborem took over a city, the buildings would automatically be added and the veil would auto-spread. After g however, the Human hyborem is given a 'choice' between razing and capturing, and the auto isn't done until after the choice. Well, for the other human players, that choice automatically defaults to 'raze' for some reason ... so if a human Hyborem chooses 'keep' there is a finite and fixable OOS (via rehosting).
For Hyborem, it is the CHOICE that causes the OOS. A choice, in retrospect, which isn't needed. Ironically I was one of the people who originally complained about Hyborem's lack of AC options, which led to the OOS-inducing change being done >.> So I can somewhat indirectly apologize for that
But yes, remove the choice, make it automatic again, and include ffs DEMON CITIZENS as one of the auto-buildings, everything should be golden.
I mean, if a city is gifted to u, u kinda have to deal with some issues I guess, but if u capture it, you should certainly have access to demon citizens. However, if u make demon citizens buildable and require fallow trait, should work. Adding 20 - 100 happiness per city as a side effect of the fallow trait might work as well. These 'quick fixes' however, only work so long as Infernals are the only fallow civ. If they aren't, you will have to make a separate fallow trait for non infernal civs.