[Immaculate]

If you click on the unit and get it's 'stat's, you'll see three lines on it at the bottom. One is red and indicates damage. One is blue and indicates fatigue and one is green and indicates poison. Poison will accumulate with successive hits or exposure and be 'applied' over successive turns. When applied, the 'green line' goes down and the unit is damage (making the red line bigger).
I think at the end of combat, poison is applied for another handful of turns and so during that handful of turns, more of that 'green line' gets turned into 'red line' (ie damage). So if there is more 'green line' then the unit has hit points, it will kill the unit.

Note that things like regeneration can counter this obviously.

Note that this effect is similar with banefire or decay aging effects i believe. you will continue to age after the battle for ~4-6 turns which can hand a unit several more afflictions or even kill a unit.


Take all this with a grain of salt; while i think its right, i don't absolutely know so.