How does Damage Reduction, Damage Resistance, and Damage Immunity Work?

This post is concerned specifically with how damage is handled regarding Reduction, Resistance and Immunity. Manny also wrote up a great guide about this as part of his Magic 101 thread, here.

Each type of damage mitigation is listed in its own section below, click on the name of the section to be taken to the NWNwikki page on that topic.

Damage Reduction:

This comes listed in the formula: Damage Reduction (+X/soak Y).

(Note: damage reduction will only apply to the 3 physical types of damage, Slashing, Piercing, and Bludgeoning, however, it does apply to all 3 types of physical damage all the time.)

X is the enhancement bonus OR attack bonus needed on an attacker's weapon to ignore the reduction.

Y is the amount of physical damage damage subtracted, or "soaked", if your reduction is not ignored by the weapon.

For example: lets say you have bracers that give +3/soak 10 damage reduction. Two enemies hit you. One hits you with a +1 mace for 8 bludegoning damage, the other hits you with a +4 Longsword for 10 slashing damage.

First, the mace. It is only a +1 weapon, so it cannot ignore your damage reduction (it must be +3 or greater to do that), so you reduce the damage by 10. The mace only did 8 bludgeoning damage, so you take nothing from the blow.

Second, the Longsword. It is +4, so it ignores your damage reduction. You take ALL 10 slashing damage.

Damage Resistance:

This comes in the formula: (Type) Resistance X/-.

(Type) is in the name of the resistance, and describes what damage type it applies to.

X is the amount of damage that is taken off all attacks of that type. There is no way around damage resistance (other than inflicting damage of another type, you cannot ignore it with a more powerful/magical weapon/attack, etc).

For Example: You have Fire resistance 10/-, and Piercing resistance 20/-. You are struck by a Burning Hands spell for 13 fire damage, an arrow for 15 piercing damage, and a punch for 5 bludgeoning damage.

First the Burning Hands spell. It does 13 fire damage, (you fail your save), so your fire resistance soaks 10 and you take only 3 points of fire damage.

Second, the arrow. It does 15 piercing damage, your piercing resistance soaks 20. You take no damage at all (the bonus of the bow or the arrow does not matter because it is a resistance, not damage reduction).

Third, the punch. It does 5 bludgeoning damage. You have no bludgeoning resistance, so you take all 5 points.

(Note: you may have resistance to any type of damage except magical in Aenea)

Damage Immunity:

This comes in the formula: (Type) Immunity X%.

(Type) is in the name of the immunity, and describes what damage type it applies to.

X is a % value that all damage of that type is reduced by.

For example: you have 75% acid immunity. You strike a wizard with the Acid Sheath spell active and the sheath inflicts 84 points of acid damage, then he hits you with a Magic Missile spell for 16 magical damage.

First the Acid Sheath. it does 84 damage, which is instantly reduce by 75% due to your acid immunity. 75% of 84 is equal to 63. So 84 - 63 = 21. You take only 21 points of acid damage.

Then the Magic Missile hits you for 16 magical damage. You take all 16 points because you have no resistance or immunity to magical damage.

(Note: You may have immunity to any type of damage except magical in Aenea. Also, Damage Immunity is applied before Resistances and Reductions. Mathematically speaking, this is a very good thing )

Stacking:

Each of the above types of damage avoidance will stack with the other 2 types. So you can stack Damage Reduction (reduction only works for physical damage types so you must also have resistance and/or immunity to physical damage types for it to stack) with Damage Resistance AND Damage Immunity.

For example, you have 20/+5 damage reduction, slashing resistance 10/-, piercing resistance 10/-, bludegoning resistance 10/-, slashing immunity 50%, piercing immunity 50%, and bludgeoning immunity 50%.

If you are struck by any physical damage from a source that is not at least a +5 weapon, you will first cut the damage in half due to your immunity, then you will subtract 10 from the remainder due to your resistance, and then you will subtract 20 from your damage reduction.

This means that being hit for 100 physical damage would become 50 damage due to your 50% immunity, then 40 due to 10/- resistance, then 20 due to 20/+5 reduction. Not too shabby, eh?

Only your highest resistance to a damage type will apply. Having multiple values of fire resistance, for example, will NOT stack. The only exception to this is in the case of the energy resistance feats, that are applied separately from other energy resistances.

Damage Immunities of the same type WILL stack with one another up to the cap of 100%.

From NWNwiki:

Damage reduction does not stack with itself, with a few exceptions. Instead, when a creature has multiple sources of damage reduction, the one that reduces the most damage among those that are not overcome by an attack's enhancement bonus will apply. If there is a tie, the oldest reduction effect is used. One exception to the stacking rule is that the epic damage reduction feats stack with perfect self. The other exception is that the sources of X/- damage reduction (barbarian-, dwarven defender- and epic- damage reduction) stack with each other.

Nice! Excellent info.

A note on immunities too: For the physical immunities (piercing, slashing, and bludgeoning), they work together, however, note that all three will work with the *lowest* immunity % of the three.

For example, if you have slashing immunity 50%, and piercing immunity 25%, you will only have an effective 25% slashing immunity.

EDR is effectively x/+21 damage reduction, not x/-, at least in our testing.

I have not tested it with Monk Perfect Self, but I think the Wiki may be wrong on that point. What we did found is that EDR did not stack with, say, Premonition (30/+5), but did stack with, for example, a Brawler's belt (5/- blunt).

Please correct me if I'm wrong, but I thought that was the finding?

The wiki says that EDR will stack with Barbarian or DD reduction, or Perfect Self. I am fairly certain this is absolutely correct, but I have not tested Perfect Self.

All damage reduction (+X/soak Y) will stack with physical Damage Resistances (X/-).

It is simply that multiple Damage Reduction sources will overlap rather than stack.

EDR will stack with Resistances, but not with sources of Reduction that are not in the stated list.

EDIT: I think the confusing part for some folks in this regard is that, for the purposes of stacking, Epic Damage Reduction still counts as Reduction (+X/soak Y) even though its description is listed with the Resistance formula (X/-).