King Slime

The King Slime is a boss that is, as its name would suggest, a very large slime; it is roughly 15 times larger than the Mother Slime (the largest ordinary slime). As the King Slime takes damage, it spawns additional ordinary Blue Slimes, and also becomes smaller until defeated. It will always drop one piece of Ninja clothes when defeated.

When the 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!"

The King Slime is considered a mini-boss in that it is a large and resilient enemy, but does not require any particular item or level of game advancement in order to spawn, and defeating it similarly does not provide access to items critical for further advancement. It also does not travel through blocks like other bosses, which creates shielding possibilities for the player that don't exist when battling other Bosses.

King Slime is immune to the Poisoned debuff.

Spawning
Inspection of source code has led to the following conclusions about King Slime spawning behavior:
 * The King Slime spawns only in areas a third of the map away from the spawn point. An easy way to determine if you are far enough from the spawn point is to check for the presence of bunnies, who spawn in the inner 2/3 of the map surrounding the spawn point. If you can see bunnies, you aren't far enough from your spawn point. Only the outer 1/3 of the map is viable for the King Slime to spawn.
 * The King Slime has a 1:300 chance to spawn, meaning a Water Candle can actually speed up the process of encountering the King Slime dramatically. Standing stationary with a Water Candle has a roughly 50% chance of spawning the King Slime per day. You can also use a Battle Potion to increase the spawn rate further.
 * The King Slime will only spawn during the day, above 0 depth and on a grass tile. It is possible for him to appear underground if these conditions are true and there is enough room for him to spawn.
 * There can only be one King Slime spawned at a time in any given world (If you attempt to spawn another, you die instantly).
 * Like other slimes, the King Slime vanishes if the player travels far enough from the enemy (for instance, if teleporting or if it is trapped somehow).
 * Goblin Scouts spawn under the same conditions as the King Slime, so you'll know if you're under the correct conditions for the King Slime to spawn if you encounter any.
 * The King Slime can also be spawned by crafting the Slime Crown which is created by combining 99 Gel with the Gold Crown at a Demon Altar.

Strategy
The King Slime behaves exactly like a normal slime, except that it spawns other slimes after being hurt by the player. Due to its predictable nature, it is easy for a new player to defeat the boss with even lower tier weapons. The only thing a new player should worry about is dodging the King Slime as it jumps about, as he deals a relatively large amount of damage per hit. To finish the boss quickly, projectiles such as Shurikens, Spiky Balls, or Bows and Arrows are recommended.

Digging a pit two blocks wide and three blocks deep is an excellent tactic for fighting the King Slime, which is too large to fit in such a small pit. Placing wooden platforms on top of the pit has the added benefit of keeping the smaller slimes out.

Also note that the King Slime's absolute highest jump can clear a 17-block-high wall, so if you're planning on trapping him somewhere, make your walls at least 18 blocks high.

For advanced players, build an arena for farming Tattered Cloth and Ninja clothes. Build a base on where Goblin scouts are spotted, make sure that there's no pools of water on either side of that point and there's no caverns below, because gold fish, slimes and goblin scouts may spawn on those crevices. Have a lava moat on each side of your base, while making sure that the lava is low enough not to destroy any items dropping in it. Have Battle Potion consumed and equip a Water Candle to increase chances of King Slime and Goblin Scout to spawn.

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.
 * This is the only boss (besides the Wall of Flesh) that does not have a hardmode version.