Immunities

Resistances

Resistances in Diablo 2 provide a reduction in damage taken from a source of the damage type. Fire Resistance decreases Fire Damage, Cold Resistance decreases Cold Damage and so on. Each 1% of a Resistance decreases the damage received from that source by 1%. Monsters with >99% Resistance are completely Immune to that damage source.

Similar monster types tend to have the same "primary" Resistance, which increases in Nightmare and Hell difficulty. Most monsters in hell have >99% Resistance of their "primary" Resistance, and this aligns with the damage type they do. Pit Lords, for example, are well known for their devastating inferno and their Fire Resistance increases from 70% in Normal, to 120% in Hell. While there are a few exceptions to the rule, once you're in Hell difficulty, every monster you encounter is Immune to at least one, and potentially multiple Damage types because they have >99% Resistance.

The two "Resistances" that have a lot of caveats are Physical and Magic. Physical Resistance is actually referred to as Damage Resist for monsters and Damage Reduction for Characters. While Characters have a cap of 50% Physical Resistance, monsters can have >99%. Magic Resistance cannot be reduced in any way, meaning a Magic Immunity cannot normally be broken.
In our Bishibosh example above, you can see that Bishibosh has 140% Fire Resistance in Normal difficulty. This leads us to a few reasonable questions.

• How would a monster have >99% Resistance in Normal?
• What is the purpose of having >100% Resistance if 100% confers complete immunity?
• What options do Characters have to reduce monster's Resistance and potentially break Immunity?

For question one we'll move to the next section where we describe Monster Modifiers and how they can confer additional Resistances to a Monster.
For questions two and three, see the next following section where we describe the different Skills that can lower Resistances of a Monster and what it takes to "Break" an Immunity.

Monster Modifiers

For a complete guide on all Monster Modifiers, see our Resource here. In this section, we'll only cover Modifiers which confer Resistances.

The monster classes which can have Modifiers include Champions, Uniques, and Super Uniques.

Champions: Have set bonuses according to what type they are. The only one which confers Resistances is the Ghostly type.
Uniques: Spawn with randomly generated Modifiers. They receive one in Normal, two in Nightmare, and three in Hell difficulty.
Super Uniques: Spawn with static Modifiers. Additionally, gaining one random Modifier in Nightmare, and two in Hell Difficulty.

In our Bishibosh example above, the reason why his Fire Resistance is so high is because of his Super Unique monster status. Bishibosh spawns with "Fire Enchanted" and "Magic Resistant" as static modifiers. Both of these confer additional Fire Resistance. See the table below for a full list of Modifiers that confer Resistances, along with how much and to which Resistance type.

1: Ghostly is a Champion affix which sets the monster's Physical Resistance to 80% instead of adding. Immune to Physical, Ghostly Champions are mislabeled.
2. A monster with a higher level conviction Aura than a Paladin's actually cancels out the Paladin's Aura. This stops conviction Aura from lowering the monster's Resistances. Other surrounding monsters without conviction or a lower level conviction are still affected.

Reducing Resistances / Breaking Immunities

Luckily for us, Diablo 2 gave us a few different avenues for reducing the Resistances of monsters. The bottom cap for any Resistance is -100%. Just as every +1% of Resistance reduces damage taken, every -1% of Resistance increases the damage taken by the monster. We can also potentially "Break" the Immunities with enough -% Resistance from Skill effects. The five types of Resistance reducing effects come from Sunder Charms, Curses, Auras, -xx% Enemy Resistance, and Pierce. They are applied to monsters in this order, and that's important to keep in mind, moving forward.
Important Note: If a monster is Immune to a damage type, effects which can lower Immune monster's Resistances are only 1/5 effective. Without Sunder Charms, some monsters simply cannot be affected by certain Damage types.

Sunder Charms

Sunder Charms are an addition in Patch 2.5 that supersedes the previous logic of Monster Resistances. If the monster was Immune to a damage type, it is no longer considered Immune and its resistance is changed to 95%. They also change how future sources of -xx% Enemy Resistance apply. coldmastery, conviction and lowerresist apply at 1/5th effectiveness if the monster is Sundered. This includes infinity's conviction Aura when wielded by your Mercenary or irongolem. While all other sources of -xx% Enemy Resistance work at 100% efficiency.

Example: Bishibosh in Hell has 175 Fire Resistance. A Fireball/Meteor Sorceress equips the flamerift. Bishibosh is no longer Immune AND has 95 Fire Resistance.

• flamerift sunders Fire Immunity
• coldrupture sunders Cold Immunity
• crackoftheheavens sunders Lightning Immunity
• rottingfissure sunders Poison Immunity
• bonebreak sunders Physical Immunity
• blackcleft sunders Magic Immunity

Auras

conviction Aura reduces a monster's Fire / Cold / Lightning Resistances by 30% starting at Skill level 1. It gains an additional 5% for each Skill Level, reaching -125% at level 20. The cap for this Skill's reduction value is -150%. When applied to a monster that is Immune, this value is only 1/5 effective. (So a max Level conviction Aura will only reduce an Immune monster's Resistance by -150%/5 = -30%)

sanctuary Aura sets an Undead monster's Physical Resistance to 0% for the purposes of applying Physical Damage from the supplier of the Aura. This does not apply for Crushing Blow calculations, so Crushing Blow damage is still negated by the monster's Physical Resistance.

Curses

Lowerresist Curse reduces a monster's Fire / Cold / Lightning / Poison Resistances by 31% starting at Skill Level 1. It gains an additional % for each Skill Level, reaching -62% at Level 20. The cap for this Skill's reduction value is -70%. When applied to a monster that is Immune, this value is only 1/5 effective. (So a max Level lowerresist Curse only reduces an Immune monster's Resistance by -70%/5 = -14%)

amplifydamage Curse reduces a monster's Physical Resistance by 100% starting at Skill Level 1. When applied to a monster that is Immune, this value is only 1/5 effective. (So any Level amplifydamage Curse only reduces an Immune monster's resistance by -100%/5 = -20%) amplifydamage applies before sanctuary Aura, making it pointless to use both effects.

decrepify Curse reduces a monster's Physical Resistance by 50% starting at Skill Level 1. When applied to a monster that is Immune, this value is only 1/5 effective. (So any Level decrepify Curse only reduces an Immune monster's resistance by -50%/5 = -10%) decrepify applies before sanctuary Aura, making it pointless to use both effects.

grimward applies a Fear Curse that reduces a monster's Physical Resistance by 20% starting at Skill Level 1. It gains an additional 5% for each Synergy point from findpotion, reaching -120% at full Synergy. When applied to a monster that is Immune, this value is only 1/5 effective. (So Level 1 grimward Fear Curse only reduces an Immune monster's resistance by -20%/5 = -4%) grimward applies before sanctuary Aura, making it pointless to use both effects.

If we take our Bishibosh example in Normal, he has 140% Fire Resistance. To break his immunity, we would need a combined -205% to reduce him to 99%.

Reduction Needed = (Resistance - 99) * 5
Reduction Needed = (140 - 99) * 5
Reduction Needed = 205%

Because of this, neither Lowerresist at a maximum of -70% nor conviction at a maximum of -150% are able to break this Immunity alone. BUT if you combine the two effects, you can successfully break the Immunity. Note: When applying to an Immune monster, even if their Resistance was "broken" by another source, effects which can break immunities are always calculated at 1/5 effectiveness.

-xx% Enemy Resistance

Items with -xx% Enemy Resistance (i.e. Rainbow Facet) only applies to monsters which are not Immune, or whose Immunity is considered broken by other sources. The final Resistance reduction is the summation of all sources equipped to the character. This Resistance reduction does not apply to damage dealt by Character summons, with the sole exception of hydra. -xx% Enemy Resistance cannot break any Immunities, regardless of the total value.

Pierce

Pierce is a unique Resistance reduction effect from coldmastery. coldmastery reduces a monster's Cold Resistance by 20% starting at Skill Level 1 for Skills used by the Sorceress herself. It gains an additional 5% for each Skill Level, reaching -115% at level 20. There is no cap to this value. Pierce cannot break Immunity, so it does not apply to monsters that are Immune to Cold, but applies if the Immunity is broken by another source.

NOTE: As of Patch 2.6 coldmastery is only 1/5th effective against monsters that have their Immunity broken by a different source. This is a MASSIVE nerf overall.

-xx% Enemy Resistance and Pierce can both apply to a target if they are not Immune, or have their immunity broken by another source. The total Resistance reduction is the summation from all sources. Reminder that this cannot lower a monster's Resistance below -100%, any reduction >199% is redundant.