Poison

Start with a Hit that has Physical or Chaos damage contributing to Poison and an explicit source of Poison chance. The base Poison deals Chaos damage per second equal to 20% of the Hit's combined pre-mitigation Physical and Chaos damage for two seconds. Poison can stack, but only the strongest instances up to your current Poison limit deal damage.

Application must be explicit

Physical and Chaos damage do not create Poison chance by themselves. The Hit needs wording such as chance to Poison, Poisons enemies, or inflicts Poison. Allowing another damage type to contribute to Magnitude still does not supply Poison chance.

Base damage is 20% per second

The base Magnitude is Chaos damage per second equal to 20% of the combined pre-mitigation Physical and Chaos damage of the Hit that inflicted it. The default two-second duration therefore deals 40% of that contributing damage before the target's later mitigation and damage-taken effects.

Pre-mitigation uses the Hit before defence

Pre-mitigation damage includes the attacker's applicable damage modifiers but is measured before the target's Armour, Resistances, Block, and damage-taken modifiers. Generic modifiers to damage you deal are not applied to the Poison again after its Magnitude is formed.

The default Poison limit is one

A character can normally have one of their Poisons dealing damage on a target at a time. Modifiers can raise that limit. Each Poison has its own duration, and only the highest-damage instances up to the applicable limit deal damage while weaker active instances wait.

External sources share your limit

The character's Poison limit also applies to external Poison sources owned by that character, including Poison Clouds. Creating several clouds does not give each cloud a separate full Poison limit on the same target.

Poison bypasses Energy Shield

Poison damage bypasses Energy Shield and removes Life directly. The Poison is still Chaos damage, so the target's Chaos Resistance and general modifiers to damage taken can change the damage that reaches Life.

Penetration does not affect Poison

Chaos Penetration is restricted to Hits and does not lower Chaos Resistance for the separate Poison damage over time. Lowering the target's actual Chaos Resistance or applying a general damage-taken modifier can affect Poison instead.

Other damage types need explicit contribution

Physical and Chaos Hit damage contribute to Poison Magnitude by default. Fire, Cold, Lightning, or all Hit damage contributes only when an effect explicitly permits it, such as Plaguefinger allowing all damage from Hits to contribute.

Poison Clouds use Poison rules

Decompose, Gas Grenade, and Gas Arrow create Poisonous gas or Poison Clouds that Poison enemies as described by each skill. Burning effects and Detonator skills can explode those clouds, but the Fire explosion is a separate damage component from the Poison.

Stored Poison is a payoff resource

Toxic Growth and Toxic Domain can store expected Poison damage on their pustules, while Plague Bearer stores a portion of expected Poison damage you deal. Stored Poison is used by those skills' own detonation or release rules and is not another Poison stack on the enemy.