Hits against any target can potentially cause a Stun on that target, depending on the damage dealt. Stunning a target interrupts their current action and prevents them from taking actions for a short time. There are two types of Stun:
Light Stuns last a fraction of a second but can be inflicted frequently. Any Hit has a chance to cause a Light Stun. The chance is based on the damage dealt, up to 100% base chance for Hits that deal 100% of the target's maximum Life. Chances lower than 15% are treated as 0%.
Heavy Stuns occur when a target's Stun bar is filled and last multiple seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).
Player Physical damage and Melee Damage each have 50% more Light Stun chance and Heavy Stun buildup. These bonuses are multiplicative with each other.
Monster Physical damage and Melee Damage have 100% more and 33% more Light Stun chance and Heavy Stun buildup respectively. These bonuses are multiplicative with each other.
Heavy Stun
Heavy Stuns occur when a target's Stun bar is filled, interrupts the target's current action and prevent them from taking actions for a few seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).
While an enemy is Heavy Stunned, they cannot act and count as Immobilised. It will also be harder to Heavy Stun them again for a short time afterwards.
Physical damage, and player (but not monster) Melee Damage, each cause 50% more Heavy Stun buildup. For players, these bonuses are multiplicative with each other.
Light Stun
Any Hit has a chance to Light Stun the target, interrupting their current action and preventing them from taking actions for a fraction of a second. The chance is based on the damage dealt, up to 100% base chance for Hits that deal 100% of the target's maximum Life. Chances lower than 15% are treated as 0%.
Physical damage, and player (but not monster) Melee Damage, each have 50% more Light Stun chance. For players, these bonuses are multiplicative with each other.
Daze
Some skills and effects have a chance to apply Daze to enemies on Hit.
Daze lasts for 8 seconds, and a Dazed enemy will take 50% more Stun Buildup. There are also a number of Skills, Effects, and other mechanics which interact with Daze for various benefits.
Player Stun Threshold
Your base Stun Threshold is equal to your maximum Life. A Hit that deals damage equal to or greater than your Stun Threshold is guaranteed to Stun you, and the chance scales down linearly for lower damage Hits. A Stun chance of less than 10% is treated as 0%.
Knockback
Knockback pushes Enemies away when Hit.
Pinned
Certain skills and effects allow damage to build up Pinned. Once this build up passes the enemy's Pinned Threshold, they are Pinned, preventing them moving, being moved or Evading for 3 seconds. They are also Light Stunned when they become Pinned.
Pinned targets count as Immobilised.
Petrify
Petrify is a Debuff which causes targets to become covered in stone and unable to move or act.
Immobilised
A target is immobilised if it cannot move, for example due to being Frozen, Pinned, Heavy Stunned, or Electrocuted.
Slow
Slows are modifiers from Debuffs that cause actions to take longer. Slows can apply to a specific stat (such as attack speed or movement speed) — if a specific type of slow is not specified, it applies to everything the affected entity does. Slows are always multiplicative with each other.
Higher Rarity enemies are less affected by Slows:
15% less Slow effect on Magic monsters
30% less Slow effect on Rare monsters
50% less Slow effect on Unique monsters
Additionally, Slows have 10% less effect on all monsters for each player in the area beyond the first and monsters cannot be slowed below 25% of their base speed.
Taunt
Enemies you Taunt can only target you, and deal 10% less damage with hits to anyone else. If not otherwise specified, Taunt lasts for 3 seconds.
Intimidate
Intimidate is a Debuff that inflicts 10% increased damage taken and 10% reduced damage dealt.
Elusive
The Elusive buff grants increased Movement Speed, and additional chance to avoid damage. The effects of Elusive are reduced over time.