Slime Rain

Slime Rain is a minor event that consists of frequent spawns of Green, Blue and Purple Slimes. It has a random chance of occuring, signaling its beginning with the message "Slime is falling from the sky!". This 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 kills. The event ends shortly after he is defeated. This event can happen simultaneously to a Blood Moon, the two series of monsters spawning together.

The event can last anywhere from 9 to 15 hours. If 150 (or 75) slimes are not killed by the end of the event, the message "Slime has stopped falling from the sky" will appear and the Slime Rain will end naturally without King Slime ever spawning. The Sticky Situation achievement will not be received if this occurs. The event can be avoided entirely this way by staying indoors or underground for long enough. Unlike with Banners, slimes killed by lava or traps will count toward the event total. However, 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: Green Slime, Pinky, Baby Slime, Black Slime, Purple Slime, Red Slime, Yellow Slime, Jungle Slime.

Slime Rain does not end if you leave the world.

A Slime Rain can only occur if at least one player in the world has over 140 life and 8 defense (except in Expert Mode, where it can occur with a significantly decreased chance before meeting the criteria). Given those criteria, a Slime Rain has ≈ 4.9% of chance of occuring in pre-hardmode world per day (≈ 3.3% in hardmode). If King Slime has not been killed yet, the probability for a Slime Rain to occur is multiplied by 2.