Acid Rain

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"A toxic downpour falls over the wasteland seas!"

Acid Rain Icon.png
Progress bar icon

The Acid Rain is a weather event added by the Calamity Mod that occurs in the Sulphurous Sea biome. It has a 1/360,000 chance of occurring every frame (1/6000 every second) once the Eye of Cthulhu has been defeated, or can be summoned manually through the use of a Caustic Tear. The natural occurrences can be prevented by using the Broken Water Filter.

The event has multiple levels of difficulty that are encountered at different stages in progression similar to the Old One's Army, with each tier unlocking after the player has defeated a specific boss. The first tier of the event will be unlocked after the Eye of Cthulhu has been defeated, the next tier will unlock after the Aquatic Scourge has been defeated and the final tier will unlock after the Polterghast has been defeated. Each tier features the introduction of new enemies as well as buffs and new behaviors to existing enemies.


Acid Rain
CharactersUnique DropsMusic
Acid EelAcid Eel
Nuclear ToadNuclear Toad

If Aquatic Scourge has been defeated:

Flak CrabFlak Crab
Irradiated SlimeIrradiated Slime
Sulphurous SkaterSulphurous Skater
Cragmaw MireCragmaw Mire(Miniboss)

If Polterghast has been defeated:

Gamma SlimeGamma Slime
Nuclear TerrorNuclear Terror(Miniboss)


The Old DukeThe Old Duke
From Acid Eels, Nuclear Toads, Radiators,
and Skyfins:
Sulphuric ScaleSulphuric Scale

From Nuclear Toads:

Caustic Croaker StaffCaustic Croaker Staff

From Radiators caught with a Bug Net:

Radiating CrystalRadiating Crystal

From Flak Crabs, Orthoceras,
Sulphurous Skaters, and Trilobites:

Corroded FossilCorroded Fossil

From Flak Crabs:

Flak ToxicannonFlak Toxicannon

From Orthoceras:

Orthocera ShellOrthocera Shell

From Sulphurous Skaters:

Sulphurous GrabberSulphurous Grabber

From Acid Eels after defeating the Aquatic Scourge:

Slithering EelsSlithering Eels

From Skyfins after defeating the Aquatic Scourge:

Skyfin BombersSkyfin Bombers
From Cragmaw Mire:
Spent Fuel ContainerSpent Fuel Container
Nuclear Fuel RodNuclear Fuel Rod

From Maulers:

Shark FinShark Fin
Sulphuric Acid CannonSulphuric Acid Cannon

From Nuclear Terrors:

Gamma HeartGamma Heart
Phosphorescent GauntletPhosphorescent Gauntlet
If Polterghast has not been defeated:
Music Box (Acid Rain - Tiers 1 and 2)Music Box (Acid Rain - Tiers 1 and 2)

If Polterghast has been defeated:

Music Box (Acid Rain - Tier 3)Music Box (Acid Rain - Tier 3)

Event Progress

Unlike other weather events, the Acid Rain has a progress bar and can be completed. To complete the event, the player must gain 90 points in the first tier, 115 in the second, and 140 in the third. In Multiplayer, the number of points required is calculated by the expression (amount in singleplayer) * ln(total players + e - 1)), rounded down, based on how many players are in the world at the start of the event.

If the player doesn't kill any enemies within the time span of four minutes, the event will automatically cancel and progress is reset for the event. The event can also be prematurely ended with a Torrential Tear.

The following is a list of enemies in the Acid Rain event and the amount of points gained from each enemy per kill. Minibosses are marked in bold.

Enemy Points per kill
Tier 1 Tier 2 Tier 3
Acid Eel Acid Eel 1 0 0
Nuclear Toad Nuclear Toad 1 0 0
Radiator Radiator 1 0 0
Skyfin Skyfin 1 1 1
Irradiated Slime Irradiated Slime n/a 0 n/a
Flak Crab Flak Crab n/a 1 1
Orthocera Orthocera n/a 1 1
Trilobite Trilobite n/a 1 1
Sulphurous Skater Sulphurous Skater n/a 1 1
Cragmaw Mire Cragmaw Mire n/a 5 4
Gamma Slime Gamma Slime n/a n/a 1
Mauler Mauler n/a n/a 4
Nuclear Terror Nuclear Terror n/a n/a 8


Any Tier

Tier 2 or 3


  • While the Acid Rain event is active, Rain will be active across the rest of the world.
  • The event's enemies will override the Sulphurous Sea's enemies, similarly to Meteor Heads.
  • Other weather events such as the Blood Moon can overlap with the Acid Rain event.
  • After the player has defeated the Eye of Cthulhu, the Acid Rain is guaranteed to begin the next time they visit the Sulphurous Sea. This only occurs once.
  • If the Eye of Cthulhu has never been defeated, the event still becomes available once Hardmode begins.


  • This event is very dangerous for early game players due to the high spawnrates of enemies.
  • Due to the event only occurring on the far side of the world, it's highly recommended the player sets a spawn close to the Sulphurous Sea or uses a Magic Conch in order to save time in the case that they die.


  • The Acid Rain's theme is corrosion!, which was composed by wiki editor RoverdriveX.
    • Its music box is titled "Acidic Downpour".
    • If the Calamity Music add-on mod is disabled, the Old One's Army's theme will play instead.
  • The final tier's theme is nuclear monsoon, which was composed by PinpinNeon
    • Its music box is titled "Sulphuric Monsoon".
    • If the Calamity Music add-on mod is disabled, the Thunderstorm's theme will play instead.
    • This theme is a remake of the final tier's previous theme, radioactive, which was also composed by PinpinNeon.
  • It is the second event added to the Calamity Mod, following the Boss Rush.
  • Some of the event's enemies are based on extinct aquatic lifeforms, such as the Orthocera and Trilobite.
  • Several designs of the event's enemies are also based on artworks submitted for a Sulphurous Sea enemy design contest on the Calamity Mod's art discord server.
  • The event used to be a variation of the Rain event in the Sulphurous Sea, similar to Blizzards in the Snow biome.
    • Prior to that, it was known as "Irradiated Rain" and replaced Rain after the Moon Lord was defeated.


These history sections are still a work-in-progress, and may not yet contain changes relevant to the current version of the Calamity Mod.
  • Updated Boss Checklist portraits for tiers 1 and 3.
    • Now plays the "Thunderstorm" music track instead of the "Solar Eclipse" music track if the Calamity Music Mod is not installed.
    • Fixed Tier 3 being displayed at the incorrect point in progression on the Boss Checklist mod.
    • Increased tier 1 kill requirement from 90 to 110, tier 2 kill requirement from 115 to 135, and tier 3 kill requirement from 140 to 170.
    • Decreased time required without killing enemies to end the event from 4 minutes to 2.5.
  • Fixed a bug that would end the event prematurely while enemies were being fought due to natural rain ending.
    • Can now occur if the player defeats Aquatic Scourge before defeating Eye of Cthulhu or Wall of Flesh.
    • Reduced the enemy kill count requirements for tiers 2 and 3.
    • If no Acid Rain enemies are killed for 4 minutes while the event is ongoing, the event will stop.
    • Tier 2 is now forced 10 seconds after defeating Aquatic Scourge.
    • Reduced the amount of kills to clear the first two tiers from 180 kills to 90 in Tier 1 and 125 in Tier 2.