Shadow Slime

The Shadow Slime Is a boss that appears after The Phantom ( https://terrariafanideas.fandom.com/wiki/The_Phantom )

When it appears there will be a chat message that says "The Slimes Have All Died, NOW THE REAL FUN BEGINS!"

To Summon him you need to kill 200 slimes and go to the end 9000000

Phase 1

He will start summon weaker versions of the phantom with 100 Hp and have very similiar moving patterns to king slime

Phase 2

There will be a chat message that says "I HAVE HAD ENOUGH OF THIS, NOW YOU WILL DIE! MORTAL!"

he will start shooting weaker versions of spirit of flight ( https://terrariafanideas.fandom.com/wiki/Spirit_of_Flight ) at the player

Phase 3

A chat message will appear saying "HOW ARE YOU STILL ALIVE? YOU WILL DIE EVEN QUICKER NOW!"

He will start summoning weaker versions of himself with 1000 HP

DEATH

A chat message will appear saying "FOOL I WILL COME BACK AS EMILS WODAHS KRAD!"

Drops

Shadow Eye

Shadow Slime pet (its one of those weaker ones mentioned in the article!)