King Slime

The  appears randomly and can be found in caves, as well as on the surface. The boss can also be spawned by using the Slime Crown, which can be crafted from 99 gel and a Gold Crown at a Demon Altar.

It has 2000 HP when spawned. When you damage it, blue slimes will break off of him and the King Slime itself progressively shrinks in size. It drops a random piece of the Ninja Set upon death.

350px

The chance of a King Slime appearing in the place of any other slime is 1:300, and holding a Water Candle will improve the spawn chance of King Slime. The King Slime is not immune to lava. Dynamite works very well against the King Slime, but is difficult to mobilize correctly as the King Slime is large and difficult to trap. King Slime has a base attack of 40.

A method to quickly spawn a King Slime is to make a brand new (preferably small) map, get to one of the edges of the home biome (You must be about 1/3 away from your respawn point) and stop in the last place with grass before a new biome begins. Using a Water Candle gives you a 0.48% higher chance of spawning a King Slime.

An equably advisable method is to build a Sky Bridge going to the very end of the world. Plant a Grass Seed and wait for it to cover the dirt, or you may use the Staff of Regrowth to save time and money. Stand at the edge of the world, this will increase the chance of the King Slime spawning as you are farther from spawn.

Tactics
An advisable strategy for beginners is to make a 10x14 box in the ground (picture to the right) and box yourself in (you may need Shiny Red Balloon, Grappling Hook/Ivy Whip, or a Cloud in a Bottle to get out,) now use the speed glitch and wait for the boss music, or hold a Water Candle, or both. When the King Slime comes, cut yourself a way out 3 blocks tall on one of the sides, and then cut away the top, all the while dodging the King Slime, wait for it to fall in, and then close off the top. Go down to where you exited the box, and start attacking it. The King Slime WILL NOT be able to damage you, and the only problem will be the blue slimes, which will easily die to the Muramasa or other swords. You can also make a 2 block hole in the the ceiling after trapping it and do this: Pull out a flail (Blue Moon, Ball O' Hurt, Sunfury) then throw it down the hole with the mouse held down. The flail will deal constant damage to the king slime, as long as you don't fall through the hole.

An alternate strategy is to make a T shaped tower at least 11 blocks tall, as shown in the picture to the right. At this height, ground based mobs can't touch you and even the King Slime can't get itself into the "airlock." However, you may need to have a weighted weapon or a boomerang, as if you stand in the airlock, the King Slime might still be able to damage you.

You can place two blocks above your head with a two block space in between and the King slime cannot reach you (however the blue slimes that are knocked off of the King Slime can still reach you). You then can swing at it from the all directions.If you go up high with your rocket/spectre boots you will take  damage from the fall and you will get hurt by landing on the king slime

Bugs

 * On Single Player, using a Slime Crown will sometimes start the battle music and produce the "King Slime has awoken!" in the dialogue box, but there will be no King Slime in the vicinity. This is usually because the boss has spawned underground, and is trapped. It is quite difficult to pinpoint his location if such is the case. Given the fact that this glitch is quite common, it is recommended to save before using a Slime Crown to minimize wasted materials. It is also recommended to use the Slime Crown on an area with even surfaces, as this glitch tends to occur the most on heavily uneven terrain. The same may happen when King Slime spawns randomly, at which point the King Slime dialogue box will appear and battle music will start, with the boss nowhere to be seen. This may cause frustrated players to use another Slime Crown, resulting in them taking thousands of damage and dying.


 * It is possible to glitch spawn the King Slime by speeding up time using the Speed Glitch. The Speed Glitch will allow you to speed up time while the Terraria window is minimized. Terraria will still be running while minimized and by listening to the music, you can determine when a King Slime has spawned. This is a very fast method to get the ninja set.


 * Another glitch is to trap the King Slime in a cell, and then create a barrier around him. Then, take sand, and place it through a single hole in the top of the cell. Once the whole thing is filled (Although probably it shouldn't be needed), he will start to clip through the ground when he jumps.


 * It is possible for two or more King Slimes to spawn at the same time; this may be a glitch.

Trivia

 * In Ninja Gaiden, one of the first bosses is a giant slimy monster that devours you. If you look closely in the center of the King Slime you'll see there is a ninja trapped inside of there. Armor that resembles Ryu's is the one that King Slime drops, so it might be a reference to that boss. While the King Slime jumps it seems like the ninja moves along with the slime.


 * King Slime was also the name of a boss in the MMORPG Maplestory, which Terraria shares other elements with.


 * There is also a rare monster called a "King Slime" in the Dragon Quest Series, although it bears little resemblance to Terraria's.