I think Frank had such a nation half-finished. "Nimh" it was called (my suggestion, ahyuck.)
In one era he would hijack MA C'tis dominion power - but with the exception of some non-inanimate undead (plague monks) none of your units were cold-blooded or disease immune
.
It was amusing, anyway.
Feedback:
Partial poison resistance works as far as I can tell, but I do believe that it rounds up, so poison resistance < 100% is no good against 1 poison damage/turn. Not sure of this, however.
I don't think you can, presently at least (and probably ever) mod negative side effects onto weapons, at least in general terms. A large area of effect melle weapon will occasionally hit the wielder (and often hit allies when used in dense formation.) I think that might be the best you can do.
The plague monk should be undead (but not inanimate or mindless), as should the censor bearer. That way, they are immune to their own disease clouds.
That's a lot of spawning. Frank had his nimhians spawn regular rats, as I recall. Of course, he deviated a lot from Warhammer.
Anyway, look forward to the mod!