Landing debuffs: Accuracy vs Resistance

The three gates, in order

When a skill tries to place a debuff such as [Decrease DEF] or [Stun], the game checks three things one after another. Failing any of them ends the attempt.

1. The skill's own chance. "Has a 75% chance of placing..." is rolled first. A skill that just says "Places a..." with no percentage passes this gate every time. Skill books of the type Buff/Debuff Chance raise this number. ACC plays no part here.
2. The blocks. The game now asks whether the target can receive this debuff at all: a boss that is immune to it, [Block Debuffs], [Stone Skin], [Intercept] stacks and similar protections. A refusal here is shown as Blocked.
3. ACC against RES. Only now does your Accuracy meet the target's Resistance, in one random roll. If you lose it, the debuff is Resisted.

A debuff that passes all three can still be refused afterwards, because the target already has a stronger one of the same kind or is carrying the maximum of 10 debuffs. That is covered in How buffs and debuffs work.

The overall chance is the gates multiplied together. A 75% skill chance against a 90% ACC vs RES landing chance gives 0.75 × 0.90, about 68%.

One roll per skill use

The first gate is rolled once for the whole use of the skill, not once per target and not once per stack. We counted 11 uses of a skill with a "75% chance of placing two Poison debuffs": 9 times both were placed, twice neither, and never exactly one. The same roll covers every enemy an area skill hits.

A skill that is worded "each hit has a 30% chance" is different: every hit is its own attempt and gets its own roll. The ACC vs RES roll (gate 3) is always separate for each target.

The ACC vs RES curve

This is the exact rule, measured from the game. First work out the gap, then the chance that the debuff is resisted:

You do not need to compute it. The shape is what matters: the resist chance never drops below 3% or rises above 97%, and the middle of the curve (30% resist) sits where the target has 30 more RES than you have ACC. It depends only on the difference, so a 30-point lead is worth the same against 100 RES as against 400 RES.

Two things stand out. Above about +30 almost nothing is gained, so extra ACC is wasted. Below about −20 the chance falls off a cliff: compared with equal ACC, being 20 under costs about 10 points of landing chance, and being 50 under costs more than 50. That is why "a little under" is a bigger problem than it sounds.

A worked example

Your champion has 200 ACC and uses a skill with a 75% chance of placing a debuff. The enemy has 225 RES.

1. Gate 1: 75% to pass.
2. Gate 2: the enemy has no immunity or Block Debuffs, so it passes.
3. Gate 3: the gap is 225 − 200 = 25, which gives a resist chance of about 23%, so 77% land.

Overall: 0.75 × 0.77, about 58% per use. If the same champion had 255 ACC instead (a 30-point lead), gate 3 would be 96% and the overall chance 72%. For reference, the Nightmare Clan Boss has 150 RES; in our fights an ACC of 149 against it was resisted 7.7% of the time. The published figures for the other difficulties run from 30 RES on Easy to 225 on Ultra-Nightmare.

What moves the gap

• Buffs and debuffs on ACC and RES. [Increase ACC] and [Decrease ACC] change your number, and [Increase RES] and [Decrease RES] change the target's. The roll uses the values at the moment of casting. Bosses that place [Decrease ACC] on your team make your own debuffs land less often.
• "Ignores X% of RES". A few skills and passives say they ignore a share of the target's RES. The game shrinks the target's RES by that share before the roll. Against 225 RES, a skill that ignores 20% treats it as 180. With 180 ACC, the landing chance goes from about 46% to 92.5%.
• Everything on the stat sheet. Gear, set bonuses, masteries and your leader's aura all feed into the ACC and RES you see. See stats.

"Cannot be resisted"

About 150 champion skills carry the words "cannot be resisted", for example a guaranteed [HP Burn] or a [Bomb]. These skip gate 3 completely: the roll never happens, whatever the RES. Some conditions come with it ("...if Zavia is on the same team", "...if the target is under a Heal Reduction debuff"), so read the whole text.

It only removes the last gate. The skill's own chance still applies, and gate 2 can still stop it. Fewer than twenty skills go further and say "cannot be blocked or resisted"; those also get past [Block Debuffs]. Even then a boss's own immunity list wins, and [Intercept] stacks and [Stone Skin] stop the control debuffs they guard against, resistable or not.

Why a boss says "Blocked", not "Resisted"

The Clan Boss, for example, is immune to a long list: [Stun], [Freeze], [Sleep], [Decrease SPD], Turn Meter reduction, cooldown increases, and more. These refusals happen at gate 2, before the ACC vs RES roll, so the game shows Blocked. Your ACC was never looked at.

Which effects need the roll

• Need ACC vs RES: debuffs placed on enemies ([Poison], [Decrease DEF], [Weaken], control effects...) and the instant effects aimed at them: removing and stealing buffs, Turn Meter reduction, cooldown increases, shortening buffs and lengthening debuffs.
• No roll: buffs and heals on your own allies, cleansing your own debuffs, damage itself, and Destroy MAX HP.