How to craft it
Demon Altar
the summon with no conditions
Most boss summons want a biome, a time of day or something dead first. This one wants none of that. Stand anywhere, use it, and King Slime arrives, which is why it is the item people reach for when they are testing a weapon or teaching someone how a boss fight works.
why you make more than one
The first kill is not the point. King Slime is the fastest boss in the game to reset and the only reliable source of a few of his drops, so crafting a small stack and running him back to back is far quicker than waiting for a Slime Rain to hand him to you. Set up on flat ground, since his teleport lands him wherever it likes and a cluttered arena turns a two-minute fight into five.
the shimmer use
Drop one into Shimmer and the Slime Rain event starts immediately, which turns the crown into an on-demand event trigger for anyone farming the rarer slime drops. Check the sky first: if the event is already running the crown just sinks and you have wasted it.