King Slime

The King Slime is a pre-Hardmode boss. Its appearance is that of a massive Blue Slime wearing a jeweled golden crown and has what appears to be a ninja inside of it. It lacks the crown on the of the game.

King Slime spawns under one of the following conditions:
 * A player uses a Slime Crown.
 * 150 slimes are defeated during the Slime Rain event (or 75 if King Slime was already slain in the world at least once).
 * Rarely in the outer lateral 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.

Manual
King Slime can be summoned manually by having a player use a Slime Crown. This can be done anywhere at any time.

Random
King Slime has a very rare chance of spawning naturally if the following conditions are met:
 * The player is located in the one of the outer lateral 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 (even on singleplayer), the boss will despawn 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

 * Piercing weapons can not only damage King Slime but kill the additional slimes it spawns.
 * The Blue Slimes it spawns can drop Hearts and 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.
 * By using a Starfury while King Slime is falling down after jumping, he can take over 50 damage for each time he is hit by the star.
 * Note that King Slime himself does not drop any Gel, but the Blue Slimes he spawns do. This makes King Slime to be a good way to farm Gel, for Flamethrower ammo and Asphalt Blocks.
 * Killing King Slime on expert mode and selling all drops gives 10 gold on average.

Trivia

 * It seems that the source of the Ninja armor is from the ninja inside King Slime. According to a developer, the trapped ninja is the Guide's brother.
 * In the versions, he 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.
 * Despite sometimes being considered the easiest boss in the game, he does the second highest damage of any pre-hardmode boss, only beatable by the Wall of Flesh (Skeletron in Expert mode).
 * King Slime is the only boss capable of spawning randomly even after it has been defeated.
 * When teleporting, King Slime drops his current crown and seemingly generates a new one upon completing the teleport.
 * According to Redigit on the terraria discord, the ninja inside is the guide's brother.