King Slime

"The lord of all things slimy."

The King Slime is a pre-Hardmode boss. He is a very large slime, roughly five times larger than the Rainbow Slime, which is the largest ordinary slime.

King Slime spawns under one of the following conditions:
 * A player uses a Slime Crown.
 * 150 slimes are killed during the Slime Rain event / 75 if King Slime was previously slain.
 * Rarely in the outer sixths of the map, during daytime, and only on grass.

As King Slime takes damage, he spawns additional ordinary Blue Slimes (and in, Spiked Slimes) and will also appear to shrink in size. He will always drop one piece of Ninja armor when defeated. When King Slime spawns, the status message "King Slime has awoken!" appears, and then once defeated, "King Slime has been defeated!"

He is immune to the Poisoned, Venom, and Confused debuffs.

Most players consider the King Slime to be the easiest boss in the game, however, with his teleporting ability added in 1.3, this matter is up for debate.

While King Slime is alive, music Boss 1 will play.

Random spawning
King Slime has a very rare chance of spawning if the following conditions are met:
 * The player is located in the one of the outer sixths of the map (the far left or far right).
 * Depth is above zero.
 * It is daytime.
 * There must be standard, green grass at its potential spawn point (off-screen; see NPC spawning)

Slime Rain
If 150 Slimes are killed by players, lava, traps and/or summoned minions during the Slime Rain event, King Slime will spawn. Defeating the boss will end the event shortly after – if all players die, the boss will despawn (except in single player where the boss will teleport itself to the new location of the player at respawn) and the event will continue for some time before ending on its own (between 9-15 in-game hours). If the King Slime has already been defeated on a world, it will only take 75 Slime kills to spawn the King Slime.

Tips
This can be used to stop the event early if the player doesn't have time to deal with it
 * Piercing weapons can not only damage King Slime but kill the additional slimes it spawns.
 * The Blue Slimes it spawns drop can drop Hearts and Mana Stars, which respectively restore 20 Health and 100 Mana.
 * Spawning King Slime while in specific areas using Slime Crowns can be a quick, easy way to farm for specific drops, such as Souls, Biome Keys, Pirate Maps, Halloween and Christmas drops or Cascade, Hel-Fire, Amarok, Yelets and Kraken yoyos, due to the high number of Blue Slimes spawned during the fight.
 * An easy way to fight the King Slime is by putting platforms over an area, grapple on to the bottom of the platforms and use a ranged or magic weapon and shoot him with it as he won't fall through platforms.
 * By using a Starfury while King Slime is falling down after jumping, it can take over 50 damage for each time it is hit by the star.
 * By summoning King Slime manually during a Slime Rain and defeating it causes the event to stop.

Trivia

 * It seems that the source of the Ninja armor is from the ninja inside King Slime.
 * In the versions, it is the only boss to drop a banner.
 * The trophy dropped by King Slime seems to be a covered in slime. The Ninjatō is a common weapon which ninjas are usually shown to carry.
 * The images for the Ninja inside the King Slime and the Crown are stored in separate image files. The King Slime's sprite without them looks similar to an enlargened sprite of a normal Blue Slime.

History
Schleimkönig Roi des gelées Nyálkakirály Король слизней 史莱姆王