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The Slime Rain is an event that can happen randomly, day or night. It can only occur if at least one player in the world has over 140 health (7 hearts) and 8 defense. The odds are ~4.9% chance per day in a pre-Hardmode world and ~3.3% Hardmode. As the name suggests, slimes will rain from the sky. Not all types of slimes will fall, however, only Green, Blue, Purple, and Pinky. After 150 slimes have been defeated, King Slime will spawn. (Kills required drops to 75 if King Slime has been defeated before.) The event ends shortly after King Slime is defeated.
The event will start when the message "Slime is falling from the sky!" is displayed, and will last until the King Slime is defeated at the end or until a certain amount of time has elapsed. When the event ends, a message will appear saying "Slime has stopped falling from the sky". The event can be avoided by staying underground until it ends.
This event can be a good grind for new players since they can get a lot of Gel for torches and be able to get Slime Hook or Slimy Saddle easily if they can defeat King Slime at the end.
If played in Expert Mode, it is possible for Slime Rain to occur before the criteria are met, although the chances of this happening are significantly lower.
If King Slime hasn't been defeated in the world yet, the odds of a Slime Rain occurring are doubled.
The Slime Rain event can also cross with other events and boss fights, such as the Goblin Army event or the Eye of Cthulhu boss.