About Heavy Stunn in POE2

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Stun

Hits against any target build up Stun. Once an enemy has more Stun than their Stun Threshold, their action is interrupted and they are Stunned for a short time.

Physical damage and Melee Damage both have higher Stun buildup.
A portion of Stun also builds towards Heavy Stun on non-players.

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Heavy Stun

Hitting an enemy builds towards a Heavy Stun based on the damage dealt. Different enemies have different Heavy Stun thresholds and durations, with very large monsters having a high Heavy Stun Threshold. Players cannot be Heavy Stunned.

While an enemy is Heavy Stunned, they cannot act and count as Immobilised.

Physical damage and Melee Damage each cause 50% more Heavy Stun buildup. These bonuses are multiplicative with each other.