King Slime

King Slime is a boss that is, as its name would suggest, a very large slime: It is roughly 5 times larger than the Rainbow Slime (the largest ordinary slime). It is summoned with the Slime Crown, at the end of a Slime Rain but can also spawn on its own rarely (but never in the center third of the map). As King Slime takes damage, it spawns additional ordinary Blue Slimes. It will always drop one piece of Ninja clothes when defeated.

When King Slime spawns, a message appears in the bottom-left corner of the screen that reads: "King Slime has awoken!" Upon defeat, a similar message appears: "King Slime has been defeated!"

It is immune to the Poisoned debuff.

Random spawning
King Slime has a 1 in 300 chance of spawning in place of another slime if the following conditions are met:
 * The player is located in the two outer thirds of the map (the far left or far right).
 * Depth is above zero. It may spawn slightly underground, though, as the average ground level is higher than that.
 * There must be standard, green grass at its potential spawn point (off-screen; see NPC spawning)

Slime Rain
After 150 slimes are defeated 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 leave and the event will continue for some time before ending on its own. If the King Slime has already been defeated on a world, it will only take 75 slime kills to spawn the King Slime.

Tips

 * King Slime can be a good way to farm Gel, eg. for Flamethrower ammo.
 * King Slime behaves exactly like a normal slime, except that he jumps much higher and farther, and spawns other slimes as he takes damage.
 * Piercing weapons can not only damage King Slime but kill the additional slimes it spawns.
 * King Slime's absolute highest jump can clear a 17-block-high wall.
 * Magic weapons work well, since the blue slimes it spawns drop Mana Stars, which refill 100 mana each.
 * If you want to come across King Slime early on in the game, build your house near the edges of the world where he can spawn frequently.
 * Spawning King Slime while in specific Hardmode areas using Slime Crowns can be a quick, easy way to farm for underground Souls and Biome Keys due to the sheer amount of blue slimes spawned during the fight.

Trivia

 * Given the presence of the ninja inside, the king slime may be a reference to Ninja Gaiden, in which one of the first bosses is a slime-like creature that devours you.
 * The King Slime is the only slime that doesn't use the slime AI.

History
史萊姆之王 Roi Slime