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, 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: Forests aren't enough like they are for regular slimes.
 * 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)
 * Chance or indefinite spawn at end of slime rain (as of 1.3)

Goblin Scouts spawn in the same locations as the King Slime, so you'll know if you're in the right place if you encounter any. A Water Candle and/or the Battle Potion can speed up the process of encountering it.

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.

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