Two out of three playable underdark races resist poison, but they're also shown using poison quite a bit. Dark-elves with spidery stuff or snake whips, for example. I think it's reasonable that the poison using races down there would work out some way to cut through resistance. So, this edge would let a poisoner make a poison that reduces immunity to resistance, or resistance to nothing, with no change for everyone else, but only if they actually concoct the poison in the underdark. Say they're mixing in rare mushrooms that only grow wild down there, or something.

It might make sense to make this a little more widely available. Maybe any poisoner could have access to it for a higher cost after passing some ammount of exploration experience?