I’ve been reading forum for a while and I pretty baffled, some of them stack some of them multiply, some don’t stack, it’s like a complete mess in a head now, I guess due to the language barrier as well. Can please someone clarify, for example, I have Will of Rattosh and Manticore devotions and Essence of Ch’thon on weapon, how do they stack? Do they?
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There’s a chapter on that
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flat RR aka without “%” icon doesn’t stack but instead takes the highest on active.
with “%” icon they start to stack with each other, that’s why it’s quite important to get as many as u can to get the best damage possible, for example… curse of frailty (from occultist skill tree) + will of rattosh + ch’thon weapon for that beefy vitality% RR reduction.
That’s how I would explain it to myself the easiest and most simplest way.
But what if one RR is flat and another is %? Do they stack and how?
You usually want at least one flat RR because that’s how i think calculation works, the rest with % just increases it (boosts it).
EDIT: I forgot to warn you that if u plan to use these rings for example… you need 100% weapon damage in order that flat RR to work:
There are couple more similar instances but usually flat RR works anyhow.
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% one applies last (after “X reduced” and “-X” ones), and is a bit tricky. If the sum of your “reduced X” and “-X” are close to enemy’s resist, then “X% reduced” RR is almost useless. But if it’s significantly higher or lower, then “X% reduced” RR has some effect too, and the higher the difference, the better effect is.
Overall, it’s nice to have %RR too, if possible, but not mandatory, unlike “X reduced” and several “-X” ones.
This is false. X% Reduced doesn’t apply last but second. X Reduced applies last. -X first.
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Really? Should’ve forgotten the proper sequence over time.
The main point still stands, though - “X% reduced” RR usefullness strongly depends on enemy resist value (much stronger, than other RR), and unlike other RR, isnt really mandatory.
I agree. I even made some kind of analysis of this in the past Flat res reduction and ways to apply (and how important is it still?) - #4 by tqFan
It’s possible the sequence you mentioned might have been considered as the proper one a long time ago 
debuffs in GD gets pretty simple when you break it down:
general debuff rules (applies to more than just RR)
A: -%(minus symbol, important) stacks infinitely from different sources
B: %reduced, you get 1, highest applies (no minus symbol)
C: “flat”/n’th reduced, you get 1, highest applies
a+b+c goes together
^above rule apply to other debuffs too
like ex OA or DA shred
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Do Elemental Storm (Rhowan’s Crown constellation) and Acid Spray (Manticore constellation) stack?
No, as they are both “reduce target’s resistance by XX” and those do not stack, only count the highest. Even if the “type of resistance” is different (here Elemental for Rhowan, all resistances for Manticore): It takes the highest of the two for any Elemental resistances, and Manticore for everything else.
It’s so freaking confusing… they have different icons and both are present on the enemy/dummy…
The most clear way, i find, to understand how it applies is what kind of stat it is (which typically comes from modding).
When something states" -20% fire resistances", its the same as a piece of armor giving 20% fire resists, but negative. This means its a defensive stat.
The other 2 RR types have a duration attached to them (x reduced over y seconds), which means they’re offensive stats (just like a x burn over y seconds).
And as is logical: add all defenses together first, then apply offensive stats. Also, because the first type is a defensive stat, you can also only find it on a (de)buff skill, never on an item itself. And once you know this, you also know how this applies to OA or DA shred or slows.
For the message I have pinned in our discord:
There are 3 types of resist reduction (pay attention to the wording), which can all apply simultaneously, but in a fixed order:
• -x% resists, this one stacks with it’s own type. Use this first (additive).
• x% reduced target resists, this one does not stack with it’s own type. Highest applies, calculate this second. (Multiplicative)
• x reduced target resists, this one does not stack with it’s own type. Highest applies, calculate this last (additive)
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I know the mechanics but as I’ve said different icon DO CONFUSE.
Not aimed directly at you, just for general info as more people look for this.
Though only one hour ago you asked if 2 abilities stack 
likely because the icon isn’t for the RR itself, but the actual debuff on the target
just like how Curse of Frailty and Devouring Swarm etc has different icons
we kinda have a similar issue on the player side where the enemy debuffs down bottom right rarely tells the whole story, and icons are ofc reused too
OK, I have read this very interesting and important thread, and I think, there are the conclusions (feel free to correct me, if I am wrong):
1a) -X% “desired” resist (stack this as much as you are able to - gear, skills, granted skills, devotions…). It works against the percentage number of the monster. Monster has 50% chaos resist, you are able to achieve -50% chaos resist ===> the monster ends with 0% chaos resist
1b) be careful with rings or any gear with the same specific skill to them (two Voidheart rings won’t decrease the Chaos Resist of enemies by -20%, as you could assume by -10% chaos res + -10% chaos res. It will proc the skill more often… so better option is to pick one Voidheart and one Combustion Band). Same skill on different gear doesn’t stack the -X% resist.
2a) X% Reduced Target’s Resistances for Y time - having only one (the highest possible)?
3a) X Reduced Target’s Resistances over Y time - having only one (the highest possible)?
Conclusion: -X% desired resist seem to be the best understandable. You can stack it, you can count with it like with regular number, you can ommit the percentage symbol, when counting with it, because it goes directly against the “accumulated” resist of the monster.
Let’s have a build, we don’t consider its [non]greatness, but for the RR topic it will serve perfectly, I think. Warlock, Level 100 (GD 1.2.0.5) - Grim Dawn Build Calculator
- -X% chaos resist (let’s assume all skills and their RR proc at the very same time)
(-20% + -10% + -25% + -35%) = -90% chaos resist - X% reduced target’s resistances over Y time = 20% over 2 seconds
- X reduced target’s resistances over Y time = 15 over 3 seconds
I don’t know the final number, I haven’t understood the final formula, but I assume it can reduce the chaos resist of the monster very effectively.
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Still confusing to me…
So… QUESTIONS:
A1 - Does [ -25% poison resist ] make a monster resist drop from 10% poison resist to -15% poison resist? Or to 7.5% poison resist?
A2 - Does [ -25% poison resist ] stack with other sources of the same or only the STRONGEST [ -X% poison resist ] apply?
B1 - Does [ 25% Reduced Target’s Poison resistance for 5 seconds ] make a monster resist drop from 10% poison resist to -15% poison resist? Or to 7.5% poison resist?
B2 - Does [ 25% Reduced Target’s Poison resistance for 5 seconds ] stack with other sources of the same or only the STRONGEST [ X% Reduced Target’s Poison Resistance for X Seconds ] apply?
C1 - Does flat [ 25 Reduced Target’s Resistance for 5 seconds ] make a monster resist drop from 10% poison resist to -15% poison resist? Or to 7.5% poison resist?
C2 - Does flat [ 25 Reduced Target’s esistance for 5 seconds ] stack with other sources of the same or only the STRONGEST flat [ X Reduced Target’s Resistance for X Seconds ] apply?
A1: -15%
A2: stacks with different sources
B1: 7.5% (and if it was -10% it would go to -12.5% if I understand it correctly)
B2: strongest only, per damage type
C1: -15%
C2: strongest only, per damage type (same as B2)
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