King Slime

King Slime is a pre-Hardmode boss. His 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. As King Slime takes damage, he spawns additional ordinary Blue Slimes (and in Expert and Master Mode) and will also appear to shrink in size.

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.

Fighting King Slime is completely optional and not required for game progression, but his defeat may reward the player with money and useful items.

Manual
King Slime can be summoned manually anywhere and at any time using the Slime Crown.

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).
 * The Depth at its potential spawn point 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 single player), 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. If King Slime is summoned manually with the Slime Crown and then subsequently killed, while the Slime Rain is active, the event will end in the same way as if the player killed the King Slime that naturally spawns at the end of the event. This can be quite helpful for powerful players who want to get the Slime Rain event over with as quickly as possible.

Tips

 * Piercing weapons can not only damage King Slime, but also 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 Biome Keys, Souls, 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.
 * King Slime is one of the most difficult bosses in the game, it is recommended to have at least 500 health and to use the Zenith in order to have a fair chance at victory.
 * Note that King Slime itself does not drop any Gel, but the Blue Slimes it 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 on average.
 * When King Slime teleports, it can become stuck inside the blocks around him, preventing its movement for a while.

Trivia

 * It seems that the source of the Ninja armor is from the ninja inside King Slime. According to Redigit, 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 enlarged sprite of a normal Blue Slime.
 * Despite sometimes being considered the easiest boss in the game, it does the second highest damage of any pre-Hardmode boss, only beaten by the Wall of Flesh in Classic Mode or 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.
 * The Spiked Blue Slime and the Pirate's Curse are the only non-invincible enemies in the game without a Bestiary entry, and the King Slime is the only boss whose summoned enemies do not have one.