Slime Rain

The minor event that rains hostile slimes and ends with King Slime.

Slime Rain is a minor event in Terraria in which Green, Blue, Cloud and Purple Slimes spawn in large numbers, along with the occasional Pinky.

Quick Facts

Type: Minor event
Boss: King Slime

Facts from the source article.

Background

Every slime spawned during the event is hostile to the player, unlike slimes that appear normally, and Green, Blue and Purple Slimes can be seen falling in the background. Defeating 150 slimes during the event summons King Slime, or only 75 if he has been beaten before. The event starts at random but can also be triggered at any time by throwing a Slime Crown into Shimmer, and it ends at once when King Slime is defeated. To happen naturally, at least one player needs more than 140 maximum health and more than 8 defense, and no other event such as a Blood Moon, Solar Eclipse or Goblin Army can be under way. It never occurs while it is raining.

What players say: Player verdict on Terraria — Rotten Tweets