Acid Rain is a Calamity Mod event, not part of unmodded Terraria. Enable Calamity, defeat the Eye of Cthulhu, then visit the Sulphurous Sea. The event may begin naturally there, or you can start it yourself with a Caustic Tear.
What you need before Acid Rain can happen
- Calamity Mod: Acid Rain is modded content; it does not occur in an unmodded Terraria world.
- Progression: Defeat the Eye of Cthulhu to unlock the first tier.
- Location: Go to the Sulphurous Sea, the biome where the event takes place.
The Calamity Mod Wiki describes the event and its trigger conditions at Acid Rain and Sulphurous Sea.
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Start it with a Caustic Tear
Using a Caustic Tear manually summons Acid Rain. This is the direct option if you do not want to wait for a natural occurrence; the Broken Water Filter’s reported effect is to prevent natural events, not to replace or disable the Caustic Tear trigger. See the Calamity Mod Wiki’s Acid Rain entry for the item and event details.
Wait for a natural event
Acid Rain can also occur naturally in the Sulphurous Sea after the required progression. The Calamity Mod Wiki reports a natural occurrence chance of 1/360,000 per frame, also expressed as 1/6,000 per second, after the Eye of Cthulhu or Wall of Flesh has been defeated. The page does not specify a mod version or publication date for these figures, and a chance per second is not a guaranteed waiting time. The Sulphurous Sea entry says the event can start there after the Eye of Cthulhu; because the two wiki descriptions differ in how they state the natural-start condition, do not treat simply arriving in the biome as a guarantee.
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A Broken Water Filter can prevent natural Acid Rain occurrences, according to the event reference. If you are trying to wait for the event, check whether one is affecting your world.
Acid Rain tiers and later progression
Acid Rain has three tiers that unlock as you advance through Calamity:
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| Tier | Unlock condition |
|---|---|
| First | Defeat the Eye of Cthulhu |
| Second | Defeat the Aquatic Scourge |
| Final | Defeat Polterghast |
After defeating Polterghast, completing the final Acid Rain can cause the Old Duke to spawn from a sulphurous tornado. The Old Duke is optional and has another summon method: fishing with Bloodworms in the Sulphurous Sea after Moon Lord. See the Old Duke reference for that boss’s conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.If Acid Rain will not start
- Confirm that Calamity Mod is enabled for the world; vanilla Terraria does not include the event.
- Go to the Sulphurous Sea.
- Check that you have defeated the Eye of Cthulhu. Later event tiers require their respective progression bosses.
- If waiting for a natural occurrence, check for a Broken Water Filter.
- Use a Caustic Tear if you want to trigger the event directly.
The event wiki page gives the flavor text “A toxic downpour falls over the wasteland seas!” but the practical trigger is the mod, location, progression, and either a natural occurrence or the Caustic Tear.
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