The poison is known as "The Black Rose", there is a side-quest in the early part of 4.0 where you have to go find this scientist who continued to work on the Black Rose after Gaius gave the orders to end all production of it and destroy all evidence of its creation. Gaius refused to use such a thing to win battles, as he really, really dislikes pointless slaughter and mass-wiping out people.
This scientist however when you find him, really has no memory, but since he was still working on it after the order was given... and thanks to his memory loss, we don't know who he was doing the work still for-- well-- until 4.4
As for 4.5 leading into 5.0, Oh yeah. We are gonna have a massive battle with the Empire, I've seen that coming for a bit myself. I'm also under the belief that we have kicked the hornets nest hard and given the Empire, thanks to Solus and the Ascians, with their puppeting, they have the Black Rose fully operational. I am fairly sure that camp was just an experiment, maybe in hopes to clean up the mess they left behind with Gaius still being alive.
So, I can see the Black Rose actually being deployed on the battlefield when the nations come knocking... and thus why there is seeming a field of death and things are so... foggy. Given from what I remember about the Black Rose as well-- it works /extremely/ fast. Like, you don't even have time to realize you been poisoned fast. It hits the body and bam, your dead. Which also seems to be the implications too on the camp.
Though if it is used in 4.5 to bring in 5.0...
That would be a terrifying cutscene to witness.
