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.

Upon completing the event, the player receives the Sticky Situation achievement.

If 150 (or 75) slimes are not killed by the player within 24 hours of the event starting, 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. If you want to battle King Slime and/or receive the achievement, make sure you or your town NPC's kill slimes - slimes killed by lava or other hazards do not count.

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.

Trivia

 * The Slime Rain is most likely a reference to the popular lets play series of Terraria by Totalbiscuit and Jesse Cox.