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.

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 debuffs can reduce enemy damage reduction:
 * Cursed Inferno decreases damage reduction by 20%.
 * Cursed Inferno reduces damage reduction differently for select post-Moon Lord bosses.
 * Betsy's Curse decreases damage reduction by 34%.
 * War Cleave decreases damage reduction by 25%.
 * Marked halves damage reduction.

NPCs with Damage Reduction
Many NPCs, including vanilla ones, have natural DR in all difficulties. DR is calculated after defense, the same as damage reduction for the player.

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 on Revengeance Mode, timed from when it spawns. If the player seems to be taking less than this time (calculated by comparing the boss's life to the time elapsed, e.g. if the boss has <50% health after half the time has elapsed or <25% health after three quarters of the time has elapsed), then the boss will gain extra DR. The DR gained is based on the boss's base DR and how much faster the player is killing the boss. DR is not reduced if the player is taking longer to kill the boss than expected.

The maximum DR gained is (1 - base DR) * 0.5. If the player has not yet defeated the boss, or has the 'Extra Damage Reduction for Bosses' configuration option set to true, the maximum DR gained is (1 - base DR) * 0.75. For example, if the boss has 30% DR, the maximum gained would be: (1 - 0.3) * 0.5 = 0.35 = 35% and (1 - 0.3) * 0.75 = 0.525 = 52.5% for both cases.

Additionally, if Providence is fought during the night, the maximum DR gained increases to (1 - base DR) * 5.

A table showing the values for the boss kill times is below:

Separate Damage Reduction Boosts
These boosts directly modify damage taken and are not an additive stat like normal DR - they are multiplicative instead. They are calculated independently to normal DR as well as each other, and are not affected by Calamity's DR scaling.

Trivia

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