Slime Rain

Slime Rain is a minor event that consists of frequent spawns of Green, Blue and Purple Slimes, most of which fall from the sky. A message will inform the player that Slimes started 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.

The event can also end after enough time has passed, however King Slime won't spawn, and the Sticky Situation achievement will not be received. You can avoid this event entirely by staying indoors or underground for long enough. A message  Slimes have stopped falling from the sky will appear to inform you that the slime rain has stopped. By using this method, you won't have to battle King Slime.

History
Pluie de Slime