Damage Reduction

Damage Reduction (DR) is a stat that decreases damage received by the player or other NPCs based on a percentage. This is different from defense, which decreases damage by flat values. Each percentage point of DR is additive, incrementing a total value that the damage then gets decreased by.

Player Damage Reduction

Vanilla changes

The Calamity Mod adjusts the effective percentage of this stat for the player with a diminishing scale, with all DR reducing effects (such as Armor Crunch) being calculated after it has been scaled. NPC DR is not affected by this diminishing scaling.

The scaling formula is: 1 - ¹⁄_{(1 + dr)}

For example, if the player has 25% DR, it will be scaled to 1 - ¹⁄_{(1 + 0.25)} = 0.2, or 20% DR.

Various changes have been made to vanilla's damage reduction boosts:

• Nerfed Ice Barrier buff from 25% DR to 15%.
• Nerfed Worm Scarf from 17% DR to 10%.

Defensive accessory modifiers now increase damage reduction:

• Hard increases DR by 0.25%.
• Guarding increases DR by 0.5%.
• Armored increases DR by 0.75%.
• Warding increases DR by 1%.

Some vanilla items have gained the ability to increase damage reduction:

• Ancient Iron Helmet increases DR by 3%.
• Beetle Wings increases DR by 5% while the player is equipping Beetle armor with Beetle Shell.
• Gold Chainmail increases DR by 5%.
• Iron armor set bonus increases DR by 6%.
• Iron Chainmail increases DR by 3%.
• Iron Greaves increases DR by 3%.
• Iron Helmet increases DR by 3%.
• Lead armor set bonus increases DR by 3%.
• Lead Chainmail increases DR by 3%.
• Lead Greaves increases DR by 3%.
• Lead Helmet increases DR by 3%.
• Leaf Wings increases DR by 5% while the player is equipping Tiki armor.
• Spectre Wings increases DR by 5% while the player is equipping Spectre armor with Spectre Hood.

List of Damage Reduction Boosts

Armor and accessories

Item		Damage Reduction	Notes
	The Community	+1.25% to 5%	Depending on bosses killed
	Corrupt Flask	+4%	While in the Corruption
	Crimson Flask	+4%	While in the Crimson
	Archaic Powder	+5%	While Underground or in the Underworld
	Giant Tortoise Shell	+5%	Disappears after taking damage for 5 seconds
	Leviathan Ambergris	+5%	When dashing
	Profaned Soul Crystal	+5%	At night or when submerged in water or honey
	Shattered Community	+5%
	Tarragon Helm	+5%
	Tarragon Helmet	+5%
	Tarragon Horned Helm	+5%
	Tarragon Mask	+5%
	Tarragon Visage	+5%
	Victide Breastplate	+5%
		+10%	When submerged in liquid
	Wulfrum Jacket	+5%
	Wulfrum armor	+5%	When transformed
	Fathom Swarmer Breastplate	+6%
	Bloody Worm Scarf	+7%
	Fearmonger Plate Mail	+8%
	Old Duke's Scales	+10%	While not exhausted
	Gem Tech armor	+12%	While the pink gem is active
	Rampart of Deities	+15%	When below 50% health
	Reaver Helm	+15%
	The Sponge	+10%	When the barrier is active
	Slagsplitter Pauldron	+10%	When dashing

Buffs and debuffs

Item		Damage Reduction
	Moonshine	+3%
	Spirit Defense	+3%
	Hallowed Defense	+4%
	Phantomic Shield	+5%
	Regeneraga	+5%
	Afflicted	+7%
	Grax Boost	+10%
	Irradiated	-10%
	Corruption Effigy	-5%
	Vulnerability Hex	-10%
	Crumbling	-45%
	Armor Crunch	-67%

Alternative Damage Reduction

Alternative Damage Reduction (Alternative DR or ADR) is a hidden player statistic separate from normal damage reduction. It directly modifies damage taken, and is affected differently by the scaling formula. The following tables show a list of damage reductions that are additive of each other, including sources inflicted to the player and external sources such as enemy debuffs.

Player Sources

Source		Damage Reduction	Contact	Projectile	Notes
	Abyssal Diving Suit	0-15%			When damage taken is above 50 while the Abyssal Diving Suit Plates buff is active. Reduces by 3% as more hits are taken, up to three times where the plates are broken and DR reduces to 0%.
	Onyx Excavator Key	10%			When moving over half of the mount's top speed while mounted.
	Tundra Leash
	Snow armor	10%			Only reduces damage taken from ice-based attacks.
	Trinket of Chi	10%			After not being previously struck for 15 seconds.
	The Evolution	15%			When struck by a projectile that has been previously reflected.
	Fearmonger armor	15%			During the Pumpkin Moon or Frost Moon while no bosses are alive.
	Shield of the High Ruler	15%			When struck by a projectile that faces the player.
	Aquatic Heart	20%			While the Ice Shield buff is active.
	Phantomic Artifact	20%			When struck with the Phantomic Bulwark active.
	Permafrost's Concoction	30%			While the Encased buff is active. Effect has a 3-minute cooldown.
	Auric Tesla Royal HelmSet bonus	50%			While the Tarragon Cloak set bonus buff is active. Effect has a 30-second cooldown.
	Tarragon HelmSet bonus
	Auric Tesla Royal HelmSet bonus	50%			While the Blood Frenzy set bonus buff is active. Effect has a 30-second cooldown.
	Bloodflare Ram MaskSet bonus
	The Bee	50%			When at full health with a 10-second cooldown.
	Flesh Totem	50%			Effect has a 20-second cooldown.
	Daedalus HelmSet bonus	50%			When reflecting a projectile.
	Lion Heart	50%			When the energy shell is active while the weapon is held.
	The Transformer	50%			Only reduces damage taken from electrical attacks and enemy bullets. A list of attacks can be seen here.
	Adrenaline Meter	50-65%			When taking damage while the meter is full. Each of Adrenaline's permanent power-ups increases value by 5%, to a maximum of 65%.
	Corruption Effigy	-5%			While the player is in range.
	Vulnerability Hex	-10%			While the player is inflicted with this debuff.
	Irradiated	-10%			While the player is inflicted with this debuff.

Non-Player Sources

Source		Damage Reduction	Contact	Projectile	Notes
	Profaned Weakness	45%			While enemies are inflicted with this debuff.
	Temporal Sadness	50%			While enemies are inflicted with this debuff.

Calculation

After the total damage reduction is calculated, several effects may take place that will modify this stat:

• If the player is inflicted with Armor Crunch, the final value is multiplied by 0.33.
• If the player is inflicted with Crumbling, the final value is multiplied by 0.55.
• If the player has normal DR stat, the final value is multiplied by (1 - DR). The DR stat used is calculated after being accounted for regular DR scaling.
• After all calculations, the final value is scaled using the same formula as the normal DR stat: 1 - ¹⁄_{(1 + dr)}

For example, if a player receives 200 contact damage from an enemy inflicted by Temporal Sadness while under the effects of Armor Crunch, Bloody Worm Scarf, Corruption Effigy, and Endurance Potion.

• First, all sources of contact damage reduction are added, including Temporal Sadness and Corruption Effigy, totaling 50% - 5% = 45%.
• Next, the value is multiplied by Armor Crunch's multiplier, totaling 45% * 0.33 = 14.85%.
• Then, the normal DR from Bloody Worm Scarf, Corruption Effigy, and Endurance Potion is accounted. The player's unscaled normal DR is 7% + 10% - 5% = 12%, and final normal DR is (1 - 1 / (1 + 0.12)) * 100% = 10.71%.
• Therefore, the alternative DR reduces to 14.85% * (1 - 0.10714) = 13.26%.
• Finally, the alternative DR goes through scaling and results in a final alternative DR of (1 - 1 / (1 + 0.13259)) * 100% = 11.71%.
• In the end, the player will take 200 * (1 - 0.1071) * (1 - 0.1171) = 158 damage.

Enemy Damage Reduction

Unlike player damage reduction, enemy damage reduction is a stat not available in vanilla, but rather added by the Calamity Mod to various NPCs including vanilla ones. Similar to the player, the enemy DR is calculated after defense.

Certain debuffs can reduce enemy damage reduction:

• Crumbling has a damage reduction multiplier of 0.88x.
• Absorber Affliction has a damage reduction multiplier of 0.8x.
• Armor Crunch has a damage reduction multiplier of 0.8x.
• Kami Flu has a damage reduction multiplier of 0.8x.
• Betsy's Curse has a damage reduction multiplier of 0.66x.
• Marked for Death has a damage reduction multiplier of 0.5x.

Certain enemies such as enraged Supreme Witch, Calamitas and the Dungeon Guardian have a 99.9999% damage reduction, effectively taking 1/1,000,000 of the original damage dealt. These enemies' damage reduction cannot be altered by any debuffs.

List of NPCs with Damage Reduction

Calamity NPCs

Image NPC DR Astrum Deus Head 20% Astrum Deus Body 10% Astrum Deus Tail 30% Stellar Culex 15% Nuclear Terror 30% Overloaded Soldier 15% Guardian Commander 30% Guardian Defender 40% Guardian Healer 20% Soul Seeker 10% Soul Seeker (Supreme) 25% Colossal Squid 5% Dragonfolly 10% Acid Eel 5% (Tier 3) Atlas 15% Blinded Angler 20% Flak Crab 20% (Tier 3) Sightseer Collider 15% Cryogen 30% (Phase 1) 27% (Phase 2) 21% (Phase 3) 12% (Phase 4) 0% (Phase 5 and 6) Cryogen's Shield 40% Sightseer Spitter 15% Brimling 15% (Normal) / 60% (Boosted) XF-09 Ares (Body) 35% XF-09 Ares Gauss Nuke 35% XF-09 Ares Laser Cannon 35% XF-09 Ares Plasma Cannon 35% XF-09 Ares Tesla Cannon 35% XS-01 Artemis (First Phase) 25% XS-03 Apollo (First Phase) 25% XS-01 Artemis (Second Phase) 25% XS-03 Apollo (Second Phase) 25% XM-05 Thanatos Head 99.99% / 0% XM-05 Thanatos Body 99.99% / 0% XM-05 Thanatos Tail 99.99% / 0% Supreme Witch, Calamitas (Phase 1) 25% Supreme Witch, Calamitas (Phase 2) 25% Catastrophe (Brother of Supreme Witch, Calamitas) 25% Cataclysm (Brother of Supreme Witch, Calamitas) 25% Nova 15% The Plaguebringer Goliath 30% Mauler 5% Cryon 10% Providence, the Profaned Goddess 30% Providence Offense 30% Providence Defender 40% Providence Healer 20% Storm Weaver Head (Armored) 99.99% Storm Weaver Body (Armored) 99.99% Storm Weaver Head (Unarmored) 20% Storm Weaver Body (Unarmored) 30% Storm Weaver Tail (Unarmored) 40% Gamma Slime 15% Brimstone Elemental (Phase 1) 15% Brimstone Elemental (Phase 2) 15% Brimstone Elemental (Phase 3) 15% Bobbit Worm 25% Mantis 15% Prism-Back 25% Profaned Energy 10% The Leviathan 35% Anahita 20% Ice Shield 50% Earth Elemental 10% The Old Duke 50% (attacking) 0% (exhausted) Sulphurous Skater 15% (Tier 3) Calamitas Clone 15% Catastrophic Construct 15% Cataclysmic Construct 15% Despair Stone 35% Skyfin 5% (Tier 2 & 3) Eidolon Wyrm Head 15% Eidolon Wyrm Body 0% Eidolon Wyrm Tail 0% Plaguebringer 20% Astralachnea 15% Astralachnea (On Wall) 15% Aries 15% Crown Jewel 10% Ceaseless Void 50% Giant Clam 30% Trilobite 25% Soul Slurper 15% Polterghast (Phase 1) 15% Necroghast (Phase 2) 20% Necroplasm (Phase 3) 50% Polterghast (Clone) 10% Phantom 10% Astral Probe 15% Clam 25% Hive 15% The Devourer of Gods Body 92.5% The Devourer of Gods Body (Final Phase) 92.5% Fusion Feeder 15% Shockstorm Shuttle 15% Desert Scourge Body 5% Desert Scourge Tail 10% Sepulcher Head 99.99% Sepulcher Body 99.99% Sepulcher Body (Weak) 99.99% Sepulcher Tail 99.99% Eutrophic Ray 5% Cloud Elemental 5% Aquatic Scourge Head 5% Aquatic Scourge Body 10% Aquatic Scourge Body (Alternate) 10% Aquatic Scourge Tail 15% Astrum Aureus 50% Scorn Eater 5% Cnidrion 5% Ravager Head 15% Ravager Body 35% Ravager Claw 15% Ravager Leg 15% Ravager Head (Phase 2) 15% Rock Pillar 30% Flame Pillar 20% Hadarian 15% Armored Digger Head 10% Armored Digger Body 20% Armored Digger Tail 30% Primordial Wyrm Head 40% Primordial Wyrm Body 0% Primordial Wyrm Body (Alternate) 0% Primordial Wyrm Tail 0% Great Sand Shark 25% Orthocera 7.5% (Tier 2) 15% (Tier 3) Yharon, Dragon of Rebirth 22% Deerclops 5% Supreme Alchemist, Cirrus (Phase 1) 25% Supreme Alchemist, Cirrus (Phase 2) 25% XB-∞ Hekate (Body) 35% XB-∞ Hekate Gauss Nuke 35% XB-∞ Laser Cannon 35% XB-∞ Hekate Plasma Cannon 35% XB-∞ Hekate Tesla Cannon 35% Blazing Eye of XB-∞ Hekate (First Phase) 25% Corrosive Eye of XB-∞ Hekate (First Phase) 25% Blazing Eye of XB-∞ Hekate (Second Phase) 25% Corrosive Eye of XB-∞ Hekate (Second Phase) 25% Spine of XB-∞ Hekate Head 99.99% / 0% Spine of XB-∞ Hekate Body 99.99% / 0% Spine of XB-∞ Hekate Tail 99.99% / 0%

Vanilla NPCs

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Reactive Damage Reduction

Reactive Damage Reduction (Reactive DR or RDR), also known as Timed Damage Reduction (Timed DR or TDR) is a mechanic exclusive to Empress of Light during the day and Providence, the Profaned Goddess during the night. It is a mechanic that gives the boss extra damage reduction if the player appears to be dealing too much damage within the intended kill time. This DR gain is calculated after everything else, meaning it cannot be affected by debuffs.

This mechanic takes into account the time elapsed and the boss's health. The time elapsed is divided by kill time for the Time Ratio value, while the boss's current health is divided by its total health for the Health Ratio value. The Empress of Light and Providence have an intended kill time of 180 and 240 seconds respectively.

The damage reduction gained is based on a boss's Base DR, which is prior to any other effects such as debuffs. The total amount of damage reduction gained is equal to:
[(1 - Base DR) * 10] - ^{[(1 - Base DR) * 10]}⁄_{(2 - Time Ratio - Health Ratio)}

Reactive DR does not occur if the sum of Time Ratio and Health Ratio is greater than or equal to 1, or if the player is in Boss Rush.

For example, suppose night time Providence reaches 70% health within 46 seconds while she is inflicted with the Marked for Death debuff:

• Providence's base DR is 30% or 0.3.
• Providence's kill time value is 240 seconds. As such, the time ratio is equal to 46 / 240 = 0.19.
• The health ratio is equal to 70% or 0.7.
• Therefore, the amount of reactive DR gained is equal to (1 - 0.3) * 10 - (1 - 0.3) * 10⁄(2 - 0.19 - 0.7) = 7 - (7 / 1.11) = 0.69 or 69%.
• Providence is also affected by Marked for Death, which multiplies damage reduction by 0.5x, from 30% to 15%.
• The two values combine into a total, instantaneous damage reduction of 84%.

Trivia

• The kill time-based Damage Reduction system is inspired by a similar mechanic from The Binding of Isaac: Rebirth.