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

The Calamity Mod adjusts the effective percentage of this stat for the player with a diminishing scale, with all DR reducing effects (such as Cursed Inferno) 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.

Certain vanilla items also have their damage reduction boosts adjusted:

• The Worm Scarf only provides a 10% damage reduction instead of 17%. This brings it in line with the Bloody Worm Scarf.
• The Frozen Turtle Shell only provides a 15% damage reduction when under 50% health instead of 25%.

List of Damage Reduction Boosts

The following include a list of items, buffs, and debuffs added in the Calamity Mod that can increase or decrease damage reduction stat.

Source	Damage Reduction (DR)	Condition
The Community	1.25% to 5%	Depending on bosses killed.
Profaned Soul Artifact	2.5% 5% 0%	When Defensive Guardian is active. When Offensive Guardian is also active. If Burnt Out is active, regardless of prior conditions.
Polaris Parrotfish	1%	When Polaris Boost buff is active.
Ethereal Extorter	3% 5%	While in The Underworld. While in the Abyss.
Tequila	3%	When Tequila debuff is active during the daytime.
Lunatic Cultist (Lore)	4%	While the Lunar Events are active.
The Absorber	5% 10%	Initially. When submerged in liquid.
Ambrosial Ampoule	5%	Always.
Bloody Worm Tooth	5% 10%	Initially. When below 50% health.
Craw Carapace	5%	Always.
Cryo Stone	5%	Always.
Evergreen Gin	5%	When Evergreen Gin debuff is active.
Leviathan Ambergris	5%	When outside of liquid.
Living Dew	5%	While in the Jungle.
Moon Lord (Lore)	5%	While gravity is flipped and the Gravity Globe is equipped.
Moonshine	5%	When Moonshine debuff is active.
Irradiated	5% -10%	With the Old Duke's lore item active Without the Old Duke lore item active
Sea Shell	5%	When submerged in liquid.
Spirit Glyph	5%	When Spirit Defense buff is active.
Titan Scale Potion	5% 15%	When Titan Scale buff is active. 10 seconds after a true melee strike during the buff.
Yharim's Stimulants	5%	When Yharim's Power buff is active.
Affliction	7%	While equipped or when Afflicted buff is active
Archaic Powder	7%	While Underground or in the Underworld.
Corrupt Flask	7%	While in the Corruption.
Crimson Flask	7%	While in the Crimson.
Hallowed Rune	7%	When Hallowed Defense buff is active.
Tequila Sunrise	7%	When Tequila Sunrise debuff is active during the daytime.
Core of the Blood God	8%	Always.
Asgard's Valor	10%	While submerged in liquid.
Asgardian Aegis	10%	While submerged in liquid.
Blazing Core	10%	Always.
Bloody Worm Scarf	10%	Always.
Gael's Greatsword	10%	When held.
Grax	10%	When Grax Boost buff is active.
The Sponge	10% 15%	Initially. When submerged in liquid.
Frigid Bulwark	15% 20%	When below 50% health. When below 15% health.
Rampart of Deities	15% 20%	When below 50% health. When below 15% health.
Leviathan and Anahita (Lore)	-5%	While not submerged in liquid.
Void of Calamity	-10%	Always.
Corruption Effigy	-20%	When Corruption Effigy buff is active.
Vulnerability Hex	-30%	Always.

Alternative Damage Reduction

Alternative damage reduction 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
Banners	75% / 50%			While the player is in range.
Triumph Potion	0-15%			Varies based on enemy health.
The Evolution	15%			When struck by a projectile that has been previously reflected.
The Transformer	50%			Only reduces damage taken from electrical attacks. A list of attacks can be seen here.
Core of the Blood God Flesh Totem	50%			Effect has a 20-second cooldown.
Auric Tesla armor (Royal Helm) Tarragon armor (Helm)	50%			While the Tarragon Cloak set bonus buff is active. Effect has a 30-second cooldown.
Auric Tesla armor (Royal Helm) Silva armor (Helm)	20%			After Silva Invulnerability is over.
Ambrosial Ampoule Honey Dew The Sponge	25%			Only reduces damage taken from bees, hornets, and their projectiles. A list of attacks can be seen here.
Eskimo armor	10%			Only reduces damage taken from ice-based attacks.
Trinket of Chi	15%			While the Chi buff is active.
Fearmonger armor	15%			During the Pumpkin Moon or Frost Moon while no bosses are alive.
Abyssal Diving Suit	15%			When damage taken is above 50 while the Abyssal Diving Suit Plates buff is active.
Aquatic Heart	20%			While the Ice Shield buff is active.
Permafrost's Concoction	30%			While the Encased buff is active. Effect has a 3-minute cooldown.
Lion Heart	50%			When the energy shell is active while the weapon is held.
The Bee	50%			When at full health with a 10-second cooldown.
Revengeance Mode	20%			Before any boss is defeated.
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%.
Onyx Excavator Key Tundra Leash	10%			When moving over half of the mount's top speed while mounted.
Leviathan and Anahita (Lore)	-5%			While the player is not submerged in liquid.
Vulnerability Hex	-30%			While the player is inflicted with this debuff.
Irradiated	-10%			While the player is inflicted with this debuff.
Corruption Effigy	-20%			While the player is in range.
Void of Calamity	-15%			n/a

Non-Player Sources

Source	Damage Reduction	Contact	Projectile	Notes
Temporal Sadness	50%			While enemies are inflicted with this debuff.
Relic of Resilience	45%			While enemies contact the shards of the bulwark.
Web Spit (Black Recluse) Pink Laser (Astral Probe, Gastropod, Probe) Frost Blast (Ice Clasper, Ice Elemental) Rune Blast (Rune Wizard) Golden Shower (Ichor Sticker) Nimbus Rain (Angry Nimbus) Stinger (Hornet) Flaming Arrow (Skeleton Archer) Deadeye Bullet (Pirate Captain, Pirate Deadeye, Tactical Skeleton) Cannonball (Pirate Captain)	25%			Only in Expert Mode and Hardmode before any Hardmode boss is spawned

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 War Cleave, the final value is multiplied by 0.75.
• 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 Blazing Core, Bloody Worm Scarf, Corruption Effigy, Endurance Potion, and War Cleave.

• First, all sources of contact damage reduction are added, including Temporal Sadness and Corruption Effigy, totaling 50% - 20% = 30%.
• Next, the value is multiplied by War Cleave's multiplier, totaling 30% * 0.75 = 22.5%.
• Then, the normal DR from Blazing Core, Bloody Worm Scarf, Corruption Effigy, and Endurance Potion is accounted. The player's unscaled normal DR is 10% + 10% + 10% - 20% = 10%, and final normal DR is (1 - 1 / (1 + 0.10)) * 100% = 9.09%.
• Therefore, the alternative DR reduces to 22.5% * (1 - 0.0909) = 20.45%.
• Finally, the alternative DR goes through scaling and results in a final alternative DR of (1 - 1 / (1 + 0.2045)) * 100% = 16.98%.
• In the end, the player will take 200 * (1 - 0.0909) * (1 - 0.1698) = 151 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:

• Kami Flu has a damage reduction multiplier of 0.8x.
• Cursed Inferno has a damage reduction multiplier of 0.8x.
• War Cleave has a damage reduction multiplier of 0.75x.
• Betsy's Curse has a damage reduction multiplier of 0.66x.
• Marked has a damage reduction multiplier of 0.5x.

Some enemies have a custom damage reduction, causing them to be partially or completely immune to certain debuffs that lower damage reduction.

• The Dragonfolly and Polterghast's damage reduction can only be affected by Cursed Inferno. Additionally, Cursed Inferno's damage reduction multiplier is modified from 0.8x to 0.9x, making the debuff half as effective.
• Providence, the Profaned Goddess's damage reduction can only be affected by Cursed Inferno.

Certain enemies such as the Ceaseless Void, Dungeon Guardian, and the Adult Eidolon Wyrm 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 Jungle Dragon, Yharon 24-26% Desert Scourge Body 5% Desert Scourge Tail 10% The Devourer of Gods Body 92.5% / 95% The Devourer of Gods Body (Final Phase) 95.5% / 97% The Leviathan 35% Anahita 20% Ice Shield 50% The Plaguebringer Goliath 40% Earth Elemental 10% Cloud Elemental 5% Cryogen 10% Cryogen's Shield 40% Calamitas (Phase 1) 15% Calamitas (Phase 2) 15% Catastrophe (Brother of Calamitas) 15% Cataclysm (Brother of Calamitas) 15% Providence, the Profaned Goddess 25% Profaned Guardian (Offense) 30% Profaned Guardian (Defense) 40% Profaned Guardian (Healer) 15% Brimstone Elemental (Phase 1) 15% Brimstone Elemental (Phase 2) 15% Brimstone Elemental (Phase 3) 15% Ceaseless Void 99.99% Profaned Guardian (Main) 40% Profaned Guardian (Rock) 25% Profaned Guardian (Crystal) 5% The Dragonfolly 10% Supreme Calamitas (Phase 1) 70% / 75% Supreme Calamitas (Phase 2) 70% / 75% (above 10% health) 84% / 90% (below 10% health) Catastrophe (Brother of Supreme Calamitas) 70% / 75% Cataclysm (Brother of Supreme Calamitas) 70% / 75% Despair Stone 35% Soul Slurper 15% Profaned Energy 10% Scorn Eater 5% Cnidrion 5% Ravager (Head) 10% Ravager (Body) 40% Ravager (Left Claw) 10% Ravager (Right Claw) 10% Ravager (Left Leg) 15% Ravager (Right Leg) 15% Rock Pillar 50% Shockstorm Shuttle 15% Sand Tortoise 25% Astral Probe 15% Armored Digger Head 15% Armored Digger Body 35% Armored Digger Tail 45% Polterghast (Phase 1) 15% Necroghast (Phase 2) 20% Necroplasm (Phase 3) 50% Soul Seeker 10% Soul Seeker (Supreme) 35% Plaguebringer 20% Sepulcher Head 99.99% Sepulcher Body 99.99% Sepulcher Body (Weak) 99.99% Sepulcher Tail 99.99% Colossal Squid 5% Mauler 5% Aquatic Scourge Head 10% Aquatic Scourge Body 5% Aquatic Scourge Body (Alternate) 5% Aquatic Scourge Tail 15% Bobbit Worm 25% Eidolon Wyrm (Juvenile) 15% Eidolon Wyrm (Adult) 99.99% Cryon 10% Phantom Debris 10% Overloaded Soldier 15% Brimling 15% (Normal) / 80% (Boosted) Great Sand Shark 25% Astrum Aureus 15% Big Sightseer 15% Small Sightseer 15% Aries 15% Atlas 15% Mantis 15% Nova 15% Astralachnea 15% Astralachnea (On Wall) 15% Hive 15% Stellar Culex 15% Fusion Feeder 15% Hadarian 15% Giant Clam 30% Prism-Back 25% Blinded Angler 20% Clam 25% Eutrophic Ray 5% Skyfin 5% (Tier 2 & 3) Nuclear Terror 40% The Old Duke 50% / 0% Cragmaw Mire 0-60% Gamma Slime 15% Orthocera 7.5% (Tier 2) 15% (Tier 3) Acid Eel 0.05% (Tier 3) Trilobite 25% Flak Crab 20% (Tier 3) Sulphurous Skater 15% (Tier 3)

Vanilla NPCs

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Reactive Damage Reduction

Every boss from both Vanilla Terraria and the Calamity Mod has a value associated with it that represents a rough estimate of how long it should take to kill outside of the Boss Rush, timed from when it spawns. If the player appears to be dealing too much damage within the intended kill time, then the boss gains extra DR. 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. Reactive DR does not apply if the sum of the two values is greater than or equal to 1, there is no kill time value assigned, or the player is in Boss Rush.

In addition, there is a multiplier that varies on certain factors. The default value of the multiplier is 1. If the player has not defeated the boss or has the "Never Weaken Reactive Boss DR" configuration option turned on, the multiplier is increased to 1.5. For Providence, the Profaned Goddess at night time, this value is increased to 10 regardless of condition.

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

For example, suppose Cryogen reaches 30% health within 36 seconds while it is inflicted with the Cursed Inferno debuff, without the player defeating it prior:

• Cryogen's base DR is 10% or 0.10.
• As the player has not defeated the boss, the multiplier is equal to 1.5.
• Cryogen's kill time value is 180 seconds. As such, the time ratio is equal to 36 / 180 = 0.20.
• The health ratio is equal to 30% or 0.30.
• Therefore, the amount of reactive DR gained is equal to (1 - 0.10) * 1.5 - (1 - 0.10) * 1.5⁄(2 - 0.20 - 0.30) = 1.35 - (1.35 / 1.50) = 0.45 or 45%.
• Cryogen is also affected by Cursed Inferno, which multiplies damage reduction by 0.8x, from 10% to 8%.
• The two values combine into a total, instantaneous damage reduction of 53%.

Kill Time Table

The following includes the kill time values for each boss. Several bosses have multiple values for different phases.

Boss	Kill Time (seconds)
King Slime	60
Desert Scourge	60
Eye of Cthulhu	90
Crabulon	90
Eater of Worlds	120
Brain of Cthulhu (Creepers)	30
Brain of Cthulhu	90
The Hive Mind (Phase 1)	30
The Hive Mind (Phase 2)	90
The Perforator Hive	120
Queen Bee	120
Skeletron	150
The Slime God (Slimes)	60
The Slime God (Core)	180
Wall of Flesh	120
Cryogen	180
The Twins	180
Brimstone Elemental	180
The Destroyer	180
Aquatic Scourge	120
Skeletron Prime	180
Calamitas (Phase 1)	20
Calamitas (Phase 2)	190
Plantera	180
Leviathan and Anahita	150
Astrum Aureus	180
Golem	150
The Plaguebringer Goliath	180
Duke Fishron	150
Ravager	180
Lunatic Cultist	150
Astrum Deus (Phase 1)	60
Astrum Deus (Phase 2)	120
Moon Lord (Head & Hands)	120
Moon Lord (Core)	240
Profaned Guardians	90
The Dragonfolly	120
Providence, the Profaned Goddess	240
Ceaseless Void (Dark Energies)	20
Storm Weaver (Naked)	90
Signus, Envoy of the Devourer	120
Polterghast	180
The Old Duke	180
The Devourer of Gods (Phase 1)	90
The Devourer of Gods (Phase 2)	180
Jungle Dragon, Yharon (Phase 1, Pre-Darksun)	180
Jungle Dragon, Yharon (Phase 1, Post-Darksun)	120
Jungle Dragon, Yharon (Phase 2)	180
Supreme Calamitas	300

Trivia

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