[Twan]

No nation as IMO caves shouldn't be treated like seas (reserved lands for nations starting there) but as swamps are (provinces with something like a quagmire effect advantaging a little some creatures/nations -also able live/start elsewhere- but not needing exceptionnal magic/pretender build to win with any nation against them).

Only two nations have full darkvision on all units (EA Abysia and Agartha) but many have units with half vision (the fey nations Tir Na Nog, Eriu ; EA Jomon, Fomoria... ; and all sea nations iirc). But death nations would be more advantaged than them by dark caves I think (if maps use them in numbers comparable with say forest or swamp).

Another option may be to simply give darkvision/blindness to more summons out of death/earth paths (ie : mindless monsters even if not undeads/statues) or create some massable summons with darkvision (cave animals) in low level nature , so as it is teh ultra-common path all nations would be able to invade caves with say giant ants or other summoned molebeast).

But anyway I think something should be done to balance the effect if darkness is added to caves (if the bug is fixed someday).