King Slime

"The lord of all things slimy."

The King Slime is a pre-Hardmode boss. As the boss name would suggest, he's a very large slime. He is roughly 5x larger than the Rainbow Slime (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.
 * Randomly in the two outer thirds of the map (never in the center third).

As King Slime takes damage, he spawns additional ordinary Blue Slimes (and in Expert mode, Spiked Slimes) and will also appear to shrink in size. He will always drop one piece of Ninja clothes 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 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 below the surface, 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
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 leave (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. Although killing the King Slime will end the event, it is not required, as the event will end naturally once it reaches its end (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

 * King Slime can be a good way to farm Gel, for Flamethrower ammo and Asphalt blocks.
 * King Slime behaves exactly like a normal slime, except that he jumps much higher and farther, can teleport if stuck, doesn't float, 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.
 * Once you get a minion (probably the Hornet Staff or Imp Staff), they can automatically kill unwanted enemies in order to make way for more potential King Slime spawns (in the appropriate locations).
 * Spawning King Slime while in specific Hardmode areas using Slime Crowns can be a quick, easy way to farm for specific drops, such as Souls, Biome Keys, Pirate Maps, or Halloween weak-enemy drops, due to the sheer amount 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.
 * Another super easy way to fight the King Slime is to build a 20-block-high vertical rope in the middle of a flat area, climbing to the top of the rope then use a ranged, magic weapon or a yoyo to hit him. Neither the King Slime nor its spawns can hit you.
 * 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.

Trivia

 * It seems that the source of the ninja armor is from the ninja inside King Slime.
 * In the console, mobile and 3DS versions, it is the only boss to drop a banner.
 * The trophy dropped by King Slime seems to be a katana covered in slime. A katana 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 seperate image files. The King Slime's sprite without them looks similar to an enlargened sprite of a normal Blue Slime.

History
史萊姆之王 Roi Slime Король слизней