Game Mechanics Guide

Physique

• +2.5 Health per every point above 50
  
• +0.05 Health regeneration per every point above 50
  
• +0.4 Defensive Ability per point

Cunning

• Roughly +1% physical damage per 2.45 points
  
• Roughly +1% pierce damage per 2.45 points
  
• Roughly +1% duration damage (only bleed and trauma) per 2.15 points
  
• +0.4 Offensive Ability per point

Spirit

• Roughly +1% magic (includes fire, cold, lightning, acid, vitality, aether, chaos) damage per 2.15 points
  
• Roughly +1% magic duration damage (burn, frostburn, electrocute, poison) damage per 2.00 points
  
• +2 Energy per every point above 50
  
• +0.009 base energy regeneration and +0.25% energy regeneration per point

Resistance
Instant type
Duration type
Relevant attribute
Physical
Physical
Internal Trauma
Cunning
Fire
Fire
Burn
Spirit
Cold
Cold
Frostburn
Spirit
Lightning
Lightning
Electrocute
Spirit
Poison & Acid
Acid
Poison
Spirit
Pierce
Pierce
---
Cunning
Bleeding
---
Bleeding
Cunning
Vitality
Vitality
Vitality Decay
Spirit
Aether
Aether
---
Spirit
Chaos
Chaos
---
Spirit

Modifiers that improve "instant" damage types won't improve corresponding "duration" types, and vice versa. For example, +%fire damage won't increase your burn damage.Percentage damage modifiers stack additively with damage bonuses from attributes, so +50% "magic" damage from Spirit and +50% fire damage from gear means +100% fire damage. This provides us with an easy way to compare gear - when only fire damage is concerned, +10% fire damage will be equal to 30 Spirit.

Every type of damage listed in the table above has a Retaliation counterpart. Retaliation damage is only improved by modifiers that expressively state that they improve Retaliation damage, e.g. +%Fire Retaliation on one of Demolitionist's skills. "Core" damage modifiers won't improve the retaliation subtype, neither would any of relevant attributes (Cunning, Spirit).

Elemental damage is a batch type. Flat elemental damage splits evenly between fire, cold and lightning. If a weapon/skill does 60 Elemental Damage it really means that damage is 20 fire, 20 cold and 20 lightning. +%Elemental Damage on the other hand works the same but isn't split, providing bonus to all three elements. +60% elemental damage means +60% fire, +60% cold and +60% lightning.

Those are also batch types. +%All Damage means every damage type from the table above, but not including a Retaliation subtype. +%All Retaliation Damage includes only Retaliation Damage subtypes.

A general formulas for Offensive (OA) and Defensive (DA) Abilities are:

Grim Dawn runs a classic system where chance to hit depends on difference between attacker's Attack Rating (in this game - Offensive Ability) and receiver's Defense Rating (in this game - Defensive Ability). What's not exactly common is that:

• if you chance to hit is below 70%, you do reduced damage even if you hit
  
• this is applied to skills and spells as well as normal attacks
  
• critical hit chance is also 100% dependant on difference between attacker's OA and receiver's DA
  
• critical damage modifier also heavily depends on this.

Equality of attacker's OA and receiver's DA results in a 90% chance to hit and 0% chance to crit.

Once chance to hit exceeds 85%, a character would be able to do critical damage. There are five tiers of critical multipliers: 1.10x, 1.20x, 1.30x, 1.50x and 2.0x. If your listed* critical chance is below 10% you would only be able to do x1.10 critical hits. Breaking a limit of 10% won't just turn your critical hits into 1.20x ones, more like you would still get majority of 1.10x and sometimes 1.20x. More in the following table:
Multiplier
How to unlock
1.10x
Chance to crit above 0%
1.20x
Chance to crit above 10%
1.30x
Chance to crit above 20%
1.50x
Chance to crit above 30%
2.0x
Chance to crit above 45%

* Why mentioning 'listed' is somewhat important is because in-game tooltip lies to you about your critical chance if your chance to hit is maxed at 100%. Let's say the game tells you that your chance to hit is 100% and chance to crit is 40%. What it actually means is you have 69.2% chance to do normal damage (meaning your actual crit chance is down to 30.8%), 7.7% chance to do a 1.10x critical hit, 7.7% chance to do a 1.20x critical hit, 7.7% chance to do a 1.30x critical hit and 7.7% chance to do a 1.50x critical hit.

If you want a more in-depth look on how the game converts OA/DA into chance to hit / crit, then you can check this[www.grimdawn.com]. It is not very useful to character optimization though. This modifier is additive with the base critical multiplier. Let's say you rolled a 1.10x crit and got a +35% Crit modifier total from whatever sources. The result is a 1.10 + 0.35 = 1.45x critical hit.
If you can't crit an enemy at all then this modifier is useless to you.

In Grim Dawn, armor is not a cumulative stat, since the game utilizes locational damage. Meaning, if an enemy shoots you in the feet, only your feet armor matters.

As said on the screenshot above, all flat sources on non-armor slots are applied to every piece. This includes skills, rings, amulets, medals and notably belts.

Armor reduces incoming physical damage by formulae

70% Armor Absorbtion means that your effective armor is 70% of the normal value.

• +%Armor will increase your Armor Rating of every armor slot.
  
• +%Armor Absorbtion will increase your Armor Absorbtion, but the modifier is actually multiplicative, not additive. +10% Armor Absorbtion means 70% * 1.1 = 77%.

Base shield parameters:

• Chance to block
  
• Amount of damage blocked
  Incoming damage will be reduced by this amount if block is successful.
  
• Recovery time
  This is basically the shield block cooldown. Shield can only block once during this time interval. This means that shield efficiency is dependant on how often do you receive damage.
  
• Damage
  As of now shield damage is seldom used. I'm aware of only two skills that use shield damage instead of main hand one - Blitz and Force Wave.

Related modifiers:

• +% Shield Block Chance
  This modifier stacks additively with base chance to block
  
• +% Shield Damage Blocked
  
• -X% Shield Recovery Time

Almost any damage source, with a few exceptions. Those exceptions are mostly environmental damage like aetherfire pools, aetherfire patches, flame aura, as well as some enemy spells that do a %health damage.

DoTs can be blocked, but the block occurs only on initial application.

When it comes to procs Grim Dawn has a rather peculiar system. In most ARPGs procs are independant, so if you, have a 20% chance to do a scissor attack and a 20% chance to do a barrel roll, then there would be an overlap when both trigger, and the game would have to decide what to do with that. This is not the case for Grim Dawn.

To clarify, there are two kinds of procs in Grim Dawn - those that chance attack animation, and those that do not. We're talking about the former here, which includes weapon techniques like Zolhan's, Markovian's Advantage, Dual Wield procs, and some other stuff; full list later. Those that are not on the list work independantly of weapon techniques.

So how does Weapon Pool System works?

Let's say you don't have any procs, so it's 100% chance to do a default attack. Let's interpret it like that: roll a number between 1 and 100, and numbers from 1 to 100 would mean a basic attack.

Now, add a 20% chance to use a Zolhan's Technique. We interpret it like that: roll a number between 1 and 100, 1-20 would mean Zolhan's Technique proc, and 21-100 would mean a default attack.

Finally, add a 20% chance to use a Markovian's Advantage. Now we got two procs. And in this case the game works like that: 1-20 would mean Zolhan's Technique proc, 21-40 would mean Markovian's Advantage proc, and 41-100 would mean a default attack. It's the key difference - if inpedendant procs would have interfered with each other, then in Grim Dawn weapon procs work together to eliminate default attack by gradually filling up it's quota.

An important conclusion is that you could easily eliminate default attacks altogether. This will happen when your accumulated chance to proc a weapon skill reach 100%.

When your accumulated chance to proc a weapon skill exceeds 100% procs will start to interfere with each other. The game resolves the 'above 100%' issue by increasing the upper limit on a roll accordingly. Let's say you've got:

• Belgothian's Shears, 20% chance;
  
• Amarasta's Quick Cut, 20% chance;
  
• Whirling Death, 20% chance;
  
• Execution, 20% chance;
  
• Markovian's Advantage, 25% chance;
  
• Zolhan's Technique, 25% chance.

Total chance to do a weapon technique attack is 130%. The game then rolls a random number between 1-130. After that it's the same as with below 100% case.

Which leads to the practical advice to not exceed a 100% limit. Let's say among your procs there's one you treasure much more than others (*cough* Execution *cough*). In that case going above 100% limit would simply reduce the chance of this desired proc to be chosen from the pool of weapon skills.

Sources:
Dev post #1[www.grimdawn.com]
Dev post #2[www.grimdawn.com]
Post explaining WPS in detail[www.grimdawn.com]

Local modifiers only affect the item they placed on. Global modifiers are wholistic - they affect the character instead of the item, and through that affect other items as well.

For example, a +10 chaos damage modifier on your weapon will affect only said weapon's damage, while +%attack speed will affect both weapons if you're dual wielding.

Based on this post[www.grimdawn.com]. I myself haven't conducted any tests on the issue.

The are three kinds of resistance reduction modifiers:

• Flat reduction (x Reduced Target's Resistance);
  
  • Found on: Plaguebearer's prefix, of the Wastes suffix, Adept's/Master's Spellblade, Oblivion relic, some item-given abilities (mostly those that buff you instead of debuffing the enemy), also Mark of Dreeg component.
    
  • Multiple sources of this type do not stack, as well as doesn't stack with itself. In the case of multiple sources the strongest will take effect.
    
  • Directly substracted from resistance.
  
  
• Negative percentage reduction (-% Resistance);
  
  • Mainly found on mastery skills, like Curse of Frailty, Night's Chill, Olexra's Flash Freeze, as well as plenty of mid- and endgame item skills like Flash Freeze (from Frozen Core and Chillsurge Ring), Unmake (Tome of Names), Warpfire (Warpfire), etc
    
  • Multiple sources of this type stack. In the case of multiple sources the strongest will take effect.
    
  • Directly substracted from resistance.
  
  
• Positive percentage reduction (x% Reduced Target's Resistance)
  
  • Just like flat type, it's mostly found on items or item skills that buff the player. For example, of the Abyss suffix, Symbol of Solael component, Eternal Band, Open Hand of Mercy, Corpse Desecrator, Essense of Ch'thon augment.
    
  • Just like flat type, multiple sources do not stack.
    
  • Multiplicative. Takes effect after two other types. Works different depending on whether the resistance positive or negative:
    
    • Positive:
      final resistance = resistance* (1 - x%)
    • Negative:
      final resistance = resistance*(1+x%)

Unless mentioned, all basic ones, which you can find in the cross-table of damage types. Crowd control effect resistances like stun, entrapment, freeze, slow etc are not affected. It is currently unknown to me if the special type of resistances (life leech, reduction of health, energy leech/burn) could be reduced in any way. 80% Fire resistance
Sources:
15 Reduced Target's Resistance for 5 seconds from Mark of Dreeg
-10% Fire Resistance from Olexra's Flash Freeze Lvl 1
20% chance of 50% Reduced target's Resistance for 5 seconds
Result: 80% - 15 - 10% = 55%, positive. 55%*(1-0.5) = 27.5% final fire resistance. 80% Fire resistance
Sources:
15 Reduced Target's Resistance for 5 seconds from Mark of Dreeg
-100% Fire Resistance from Olexra's Flash Freeze Lvl 12
20% chance of 50% Reduced target's Resistance for 5 seconds
Result: 80% - 15 - 100% = -35%, negative. -35%*(1+0.5) = -52.5 final fire resistance.

A special type of modifier, could be either gear-based or skill-based. Gear-based modifier will convert physical damage of a weapon it's placed on to another type (if it's not on a weapon/shield it will still be applied to both your main-hand and off-hand, with focuses being exception for obvious reason).

The conversion happens before damage modifiers are applied, so, e.g., if you convert X% of Physical Damage to Fire Damage, +%fire damage and bonus from spirit will be applied, while +%physical damage and bonus from cunning will not.

Different sources of conversion stack additively. I don't know for certain what will happen if you go over 100% (or if it's even possible), but given how weapon pool system works and how developers like weight-based implementations, perhaps it works the same for conversion.

Some weapons have innate armor piercing. Even though it's de facto conversion of weapon physical damage to pierce damage, the game doesn't treat is as such. Armor piercing calculation happens after damage conversion, using the remaining physical damage.