[Sombre]

All immunity to darkness from being demons, blind, undead, darkvision etc would work. Partial dv would actually work pretty well too, it would be a big advantage over units with no dv at all, though demons and undead would cause trouble.

I don't know why people would want to resist this. None of the default maps even use caves to my knowledge and this is listed as a bug, Illwinter have said they've fixed it/want to fix it a couple of times.

Really this is more for user created maps than anything else, because only mappers actually use caves and they are very rarely extensive.

I haven't seen darkness autocast by any of the forts that are supposed to (though I haven't tested) or cave provinces. It should certainly be cast by all of them.