Slime Rain is a minor event that consists of frequent spawns of Green, Blue and Purple Slime and rarely Pinky. It occurs randomly, signaling its beginning with the message "Slime is falling from the sky!". Slime Rain continues until 150 slimes have been defeated, at which point King Slime will spawn. If King Slime has previously been defeated, he will spawn after only 75 slimes have been defeated. The event ends shortly after he is defeated or despawns. All slime spawns from the event will be continuously hostile, unlike naturally spawned slimes.
A Slime Rain can only occur if at least one player in the world has over 140 maximum health and 8 defense (except in Expert Mode, where it can occur before meeting the criteria – with a significantly decreased chance). Given those criteria, a Slime Rain has an approximate 4.9% of chance of occurring in a pre-Hardmode world per day (approx. 3.3% in Hardmode). If King Slime has not been killed yet, the probability for a Slime Rain to occur is doubled.
While a Slime Rain is active, Green, Blue, and Purple Slimes will appear to fall in the background. The event can last anywhere from 9 to 15 in-game hours. If the player fails to defeat 150 / 75 slimes by the end of the Slime Rain, the message "Slime has stopped falling from the sky." will appear and the Slime Rain will end naturally without King Slime spawning. The event does not end if the player leaves the world.
- Only Blue Slimes (including those spawned from a Slime Statue) and any of its variants that share the primary NPC ID value of 1 count toward the event total: Green Slime, Pinky, Baby Slime, Black Slime, Purple Slime, Red Slime, Yellow Slime, and Jungle Slime.
- Slimes killed by lava or traps will count toward the event total.
- If King Slime is summoned via Slime Crown during the event and killed, the event will end.
- In the event that the player is killed during the Slime Rain, there is a chance that King Slime will not despawn, especially if it is penned in by terrain or blocks. When the player respawns, King Slime will usually immediately teleport to the player's location if this has happened.
- In the event that the player is killed during the Slime Rain and King Slime does successfully despawn after hopping away, it is possible to continue defeating Slimes and to cause King Slime to respawn. This means that the player must defeat another 150 Slimes (or 75 Slimes if King Slime has been defeated previously) before the Slime Rain ends.
- The Slime Rain can happen simultaneously to a Blood Moon, and it can also happen while a boss is alive.
- The Sticky Situation achievement will not be awarded if the event ends without King Slime having been defeated.
- If the player is in a biome that features special variants of slimes, those slimes will not rain from the sky and their normal spawn rates will not be affected.
- Slimes spawned by the Slime Rain are not affected by the enemy spawn rate slider in Journey Mode.
- The event can be avoided entirely by staying indoors or underground, as it can end without any player interaction after a certain amount of time has passed.
- The high spawn rate of Slimes during a Slime Rain, including Pinky, greatly facilitates farming for Gel, Pink Gel, Slime Staff, and the Slimes' Banners. King Slime will also offer loot that is useful at the beginning of the game.
- An enclosed shelter with a 1×1 hole in the wall will allow the player to fire arrows and use Yoyos from relative safety. However, once King Slime spawns, it is advised to leave, as he can teleport into the shelter and trap the player.
- The spawn rate can be increased with a Water Candle and Battle Potion. Similarly, it can be decreased with Sunflowers, a Calming Potion, and Peace Candles.
- The Slime Rain effect in the background can be seen before the event is announced if the player goes up high enough.
- Some slimes appear to spawn in the air above the player which results in them falling from the sky, giving the term "rain" a literal meaning.
- The Slime Rain is one of two events in Terraria that cannot be summoned via the use of an item, the other being the Martian Madness.
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