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.
Spirit Glyph	3%	When Spirit Defense buff is active.
Tequila	3%	When Tequila debuff is active during the daytime.
Hallowed Rune	4%	When Hallowed Defense buff is active.
Archaic Powder	5%	While Underground or in the Underworld.
Bloody Worm Tooth	5% 10%	Initially. When below 50% health.
Craw Carapace	5%	Always.
Evergreen Gin	5%	When Evergreen Gin debuff is active.
Honey Dew	5%	While in the Jungle.
Leviathan Ambergris	5%	When outside of liquid.
Living Dew	5%	While in the Jungle.
Moonshine	5%	When Moonshine debuff is active.
Phantomic Artifact	5%	When Phantomic Shield buff is active.
Sea Shell	5%	When submerged in liquid.
Titan Scale Potion	5% 15%	When Titan Scale buff is active. 10 seconds after a true melee strike during the buff.
Yharim's Stimulants	6%	When Yharim's Power buff is active.
Ambrosial Ampoule	7%	Always.
Affliction	7%	While equipped or when Afflicted buff is active
Corrupt Flask	7%	While in the Corruption.
Crimson Flask	7%	While in the Crimson.
Tequila Sunrise	7%	When Tequila Sunrise debuff is active during the daytime.
Core of the Blood God	8%	Always.
The Absorber	10% 15%	Initially. When submerged in liquid.
Blazing Core	10%	Always.
Bloody Worm Scarf	10%	Always.
Grax	10%	When Grax Boost buff is active.
The Sponge	15% 20%	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.
Irradiated	-10%	Always
Void of Calamity	-10%	Always.
Corruption Effigy	-20%	When Corruption Effigy buff is active.
Vulnerability Hex	-10%	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.
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%.
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 and enemy bullets. 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) Bloodflare armor (Ram Mask)	50%			While the Blood Frenzy set bonus buff is active. Effect has a 30-second cooldown.
Honey Dew	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.
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.
Daedalus armor (Helm)	50%			When reflecting a projectile.
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.
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.

Certain enemies such as enraged Supreme 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 Jungle Dragon, Yharon 22% Desert Scourge Body 5% Desert Scourge Tail 10% The Devourer of Gods Body 92.5% The Devourer of Gods Body (Final Phase) 92.5% The Leviathan 35% Anahita 20% Ice Shield 50% The Plaguebringer Goliath 30% Earth Elemental 10% Cloud Elemental 5% Cryogen 10% Cryogen's Shield 40% Calamitas 15% Catastrophe (Brother of Calamitas) 15% Cataclysm (Brother of Calamitas) 15% Providence, the Profaned Goddess 30% Profaned Guardian (Offense) 30% Profaned Guardian (Defense) 40% Profaned Guardian (Healer) 20% Brimstone Elemental (Phase 1) 15% Brimstone Elemental (Phase 2) 15% Brimstone Elemental (Phase 3) 15% Storm Weaver Head (Armored) 99.99% Storm Weaver Body (Armored) 99.99% Ceaseless Void 50% Profaned Guardian (Main) 30% Profaned Guardian (Rock) 40% Profaned Guardian (Crystal) 20% The Dragonfolly 10% Supreme Calamitas (Phase 1) 25% Supreme Calamitas (Phase 2) 25% Catastrophe (Brother of Supreme Calamitas) 25% Cataclysm (Brother of Supreme Calamitas) 25% Despair Stone 35% Soul Slurper 15% Profaned Energy 10% Scorn Eater 5% Cnidrion 5% Astrum Deus Head 20% Astrum Deus Body 10% Astrum Deus Tail 30% Ravager Head 15% Ravager Body 35% Ravager Claw Ravager Claw 15% Ravager Leg Ravager Leg 15% Ravager Head (Phase 2) 15% Rock Pillar 30% Flame Pillar 20% Shockstorm Shuttle 15% Sand Tortoise 25% Astral Probe 15% Armored Digger Head 15% Armored Digger Body 25% Armored Digger Tail 35% Polterghast (Phase 1) 15% Necroghast (Phase 2) 20% Necroplasm (Phase 3) 50% Polterghast (Clone) 10% Phantom 10% Soul Seeker 10% Soul Seeker (Supreme) 25% 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 5% Aquatic Scourge Body 10% Aquatic Scourge Body (Alternate) 10% Aquatic Scourge Tail 15% Bobbit Worm 25% Cryon 10% Phantom Debris 10% Overloaded Soldier 15% Brimling 15% (Normal) / 60% (Boosted) Great Sand Shark 25% Eidolon Wyrm Head 15% Eidolon Wyrm Body 0% Eidolon Wyrm Tail 0% Adult Eidolon Wyrm Head 40% Adult Eidolon Wyrm Body 0% Eidolon Wyrm Body (Alternate) 0% Adult Eidolon Wyrm Tail 0% Astrum Aureus 50% 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% Crown Jewel 10% Skyfin 5% (Tier 2 & 3) Nuclear Terror 30% The Old Duke 50% / 0% Cragmaw Mire 0-50% Gamma Slime 15% Orthocera 7.5% (Tier 2) 15% (Tier 3) Acid Eel 5% (Tier 3) Trilobite 25% Flak Crab 20% (Tier 3) Sulphurous Skater 15% (Tier 3) 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%

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 under the following conditions:

• If the sum of the two aforementioned values is greater than or equal to 1
• There is no kill time value assigned
• The player is in Boss Rush.
• Any boss of the higher assigned tier is defeated. If the boss is in tier 6, defeating it will permanently disable Reactive DR for that boss.
  • The only exception is Providence, the Profaned Goddess at night or in Malice Mode.

In addition, there is a Scale Factor that varies on certain conditions. The default value of the multiplier is 1.5, which increases to 2 in Malice Mode. Certain bosses have a higher multiplier, including:

• XM-05 Thanatos: 2
• Aquatic Scourge, Astrum Deus, The Destroyer, and Storm Weaver: 3
• Aquatic Scourge, Astrum Deus, Desert Scourge, The Destroyer, and Storm Weaver in Malice Mode: 4
• Providence, the Profaned Goddess at night or in Malice Mode: 10

In Normal Mode and Expert Mode, the scale factor is multiplied by 0.67 and 0.85 respectively.

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) * Scale Factor] - ^{[(1 - Base DR) * Scale Factor]}⁄_{(2 - Time Ratio - Health Ratio)}

For example, suppose Cryogen reaches 30% health within 36 seconds while it is inflicted with the Cursed Inferno debuff, in Revengeance Mode:

• Cryogen's base DR is 10% or 0.10.
• The multiplier is equal to 1.5, as it is not in Malice Mode.
• 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%.

Timed Aggression

All bosses in Revengeance Mode or higher become more aggressive and enter their later phases regardless of their current HP if the player takes too long to defeat them. The condition for this to activate is for the player to be calculated to take 50% longer than the expected kill time, or higher.

For example, King Slime has an expected kill time of 60, and summons the Crown Jewel at 50% of its health. This means it is expected that King Slime will summon the Crown Jewel at 30 seconds into the fight. However, if the player takes at least 50% longer (i.e. King Slime is still not below 50% at 45 seconds into the fight), it will enter that phase regardless, and summon the Crown Jewel.

Kill Time Table

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

Boss	Tier	Kill Time (seconds)
King Slime	1	60
Desert Scourge	1	60
Eye of Cthulhu	1	90
Crabulon	1	90
Eater of Worlds	1	120
Brain of Cthulhu (Creepers)	1	30
Brain of Cthulhu	1	90
The Hive Mind	1	120
The Perforator Hive	1	120
Queen Bee	1	120
Skeletron	1	120
The Slime God (Slimes)	1	90
The Slime God (Core)	1	180
Wall of Flesh	1	120
Cryogen	2	180
The Twins	2	180
Brimstone Elemental	2	180
The Destroyer	2	180
Aquatic Scourge	2	120
Skeletron Prime	2	180
Calamitas	2	240
Plantera	2	180
Leviathan and Anahita	3	180
Astrum Aureus	3	180
Golem	3	150
The Plaguebringer Goliath	3	180
Duke Fishron	3	150
Ravager	3	180
Lunatic Cultist	3	150
Astrum Deus (Phase 1)	3	60
Astrum Deus (Phase 2)	3	120
Moon Lord (Head & Hands)	3	120
Moon Lord (Core)	3	240
Profaned Guardians	4	90
The Dragonfolly	4	120
Providence, the Profaned Goddess	4	240
Ceaseless Void (Dark Energies)	5	20
Ceaseless Void	5	180
Storm Weaver	5	135
Signus, Envoy of the Devourer	5	120
Polterghast	5	180
The Old Duke	5	180
The Devourer of Gods (Phase 2)	5	240
Jungle Dragon, Yharon	6	255
Exo Mechs	6	360
Supreme Calamitas	6	300
Adult Eidolon Wyrm	6	300

Trivia

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