Summoned said:
What kind of damage is the initial spell? Somehow I doubt that it does only poison damage, since when she casts it, you take much more damage than the poison over duration, because poison damage is supposed to be consistent over time, not a curve. I suspect it might be either physical or magic.
I've noticed this "initial hit" thing with poison attacks before. There is some amount of damage that is applied on every hit by a poison attack and that doesn't seem to be part of the damage over time.
I'll try to explain what I mean. As far as I understand, poison is supposed to do its damage over time and any repetition of said poison resets the duration. On top of that, poison cannot, by itself, kill a player, it will only take you to 1 life. However in practice this is not what I have observed with some poison attacks. First you will often see a sudden drop in the life orb every time a poison shot hits, then it resumes normal drain speed, as if a lot of damage was applied all at once at the time of impact. My guess is that some monsters' poison also have a bit of initial physical damage added but that's only a guess.
The first time I saw this "bug" (assuming it's one) in action was in classic in the 1.06 patch. It happened mainly with the poison potion throwing cats in act 2. Their potion was even capable of killing a character as if it wasn't just poison involved.
Andariel (1.10 this time) has the very same property in her poison spray and poison shot attacks. If you're at 1 life (presumably from previous poison attacks) and she hits you with more poison you will die. I know I've tried. I made sure nothing else cause my death, it wasn't a melee hit, it wasn't the little fires or a fallen I forgot to kill, I died the instant I was hit by the poison spray. So whenever you're hit by a poison attack whether first time or when the counter resets, you are hit by some initial damage that doesn't seem to follow normal poison rules. Maybe this damage once amplified by her bonus is what's damaging mercs so much.
Of course the above is from observations only, I don't have anywhere near the skills to go look what actually happens in the code.
As a last note, I've also seen this happen with poison skeletal magi (the ones the necro summons). That would mean having more than 1 poison mage is not a complete waste thanks to the inital hit thing.