I’m investigating this item: Serpent's Milk
It’s usefulness depends on some questions, and some math I’m unsure of:
It seems unclear whether pet poison synergises with player poison. Does 20 stacks of pet poison apply a 100% increased poison damage only from pets, or will it apply a 100% increased poison damage from both pets and the player?
Now to the math. Let’s say a single hit applies one stack at 7 DPS. Is it correct to say the item would now apply 3 stacks, then reduce the pet damage as shown in line 2 here?
If so, then Serpents milk is reducing minion poison damage to about 30% of the same hit without the item equipped. The loss to minion damage is more marked if minions already have a high chance to poison. For example, with 1000% existing poison chance, equipping the item drops minion damage to about 12% of the same hit without the item:
Now, say minions can hit 10 times rapidly. With serpents milk this makes 30 stacks of poison, and the increased player damage from 30 poison stacks (150%) starts to get a little more exciting compared with the 50% increased damage your minions would provide without the item.
But, this only works if minion poison stacks can help increase player poison damage.
Based on all this, it would seem serpents milk can be a build defining item if
a) You can get lots of minions hitting rapidly, but those minions do not do much poison damage
b) You can get a nice source of poison damage you apply yourself
c) You can easily scale increases to your damage, but cannot easily scale increases to minion poison damage, minion poison chance, or poison shred.
example 1 - Acolyte with wandering spirit or hungering soul poison being magnified immensely by a swarm of other minions applying tons of poison stacks
example 2 - Shaman’s storm totems applying lots of poisons ahead of the shaman coming in and applying poison of their own, say with serpent strike