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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 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.

Unlike other weather events, the Acid Rain has a progress bar and can be completed. To complete the event, the player must kill 90 enemies in the first tier, 125 in the second, and 180 in the third. The event can also be prematurely ended with a Torrential Tear or the non-consumable variant of the Caustic Tear. In Multiplayer, the players must kill the amount of enemies calculated by the equation (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.

Content[edit | edit source]

Acid Rain
Characters Unique Drops
Acid EelAcid Eel
Nuclear ToadNuclear Toad
Water LeechWater Leech

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, Radiators,
and Skyfins:
Sulphuric ScaleSulphuric Scale

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 RodNuclear Rod

From Nuclear Terrors:

Gamma HeartGamma Heart

From The Old Duke:

Insidious ImpalerInsidious Impaler
Fetid EmesisFetid Emesis
Septic SkewerSeptic Skewer
Vitriolic ViperVitriolic Viper
Cadaverous CarrionCadaverous Carrion
Toxicant TwisterToxicant Twister
Old Duke's ScalesOld Duke's Scales
Old Duke MaskOld Duke Mask
Old Duke TrophyOld Duke Trophy
Old Duke (Lore)Old Duke (Lore)(Always on first kill)
Treasure Bag (The Old Duke)Treasure Bag(The Old Duke)

Notes[edit | edit source]

  • 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.

Tips[edit | edit source]

  • 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 in order to save time.

Trivia[edit | edit source]

  • 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 radioactive, which was composed by Pinkie Poss.
    • Its music box is titled "Sulphuric Monsoon".
  • 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.