Acid Rain

"A toxic downpour falls over the wasteland seas!"

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.

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, 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 or the non-consumable variant of the Caustic 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.

Any Tier

 * The Sea King will begin selling Caustic Tears.
 * Abyssal Crates will contain Sulphuric Scales.

Tier 2 or 3

 * Abyssal Crates will contain Corroded Fossils, Belching Saxophone, Flak Toxicannon, Nuclear Rod, Skyfin Bombers, Slithering Eels, Spent Fuel Container, and Sulphurous Grabber.

Tips

 * 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 in the case that they die.

Trivia

 * 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 Pinkie Poss.
 * Its music box is titled "Sulphuric Monsoon".
 * If the Calamity Music add-on mod is disabled, the Solar Eclipse's theme will play instead.
 * This theme is a remake of the final tier's previous theme, radioactive, which was also composed by Pinkie Poss.
 * 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.