[Wauthan]

As far as I can tell, after some madcap Toxic Avenger campaigns, the green bar indicates that amount of HP that will be lost due to poisoning. A full bar means that the unit is doomed and will die sooner or later that battle. The "sooner or later" depends on the strength of the poison. I'm sure you've gone up against tritons and lost a bunch of units all at once as the battle ends. That's what happens when a unit gets hit by numerous blows from weak poison. Compare that to a Wyrm Pretender that usually drops an enemy in one hit, or it dies the next round. That's death poison at work.

Things can get pretty confusing if you mix a lot of different poison sources in a battle. Units drop very randomly indeed so there's surely a number of dice involved (most likely 2d6+x where x is the strength of the poison).