From the deepest swamps of Minecraft, a king has appeared.. 

While taking damage, the Slime King will alternate between two attacks with a 7 second delay between the attack ending and the next starting.

The first attack consists of him leaping high into the air before landing and throwing slime blocks in all directions. If a block hits a player, the player is grounded with sprint disabled and unable to move for a period of time. If the player is hit with multiple blocks, the player’s unable to move timer will increase by 10 seconds for every block he is hit by. When a slime block hits the ground it splits into slime balls. If a player runs over a slime ball he will receive slowness 2 for 5 seconds.

The second attack he will rapidly fire slime balls at all players in line of sight. Each slimeball deals .25hp giving a maximum potential damage of 12.5 (6.25 hearts). The slime balls have Armour Piercing. Slime balls also deal an immense amount of delayed knock back. For every slime ball that hits a player, the amount of knockback they take is increased at the end of the attack. After all slimeballs have hit or missed, players will take the knockback in accordance to the amount of slime they were hit with.

Every 500 HP dealt to the slime king decreases in size by 5 slime levels. The starting size of the Slime King is 30 (he is a big boy!). At size 3-5, 10 slimes spawn around him (Slimelings*) and target players.
This process stops after the 1500 HP mark (meaning it does split again at 1500 but not again after this).

When the Slime King hits 1000 HP it enters it’s final state and stops taking any damage. It rises 15 blocks into the air and explodes sending slime and slime blocks everywhere around him dealing damage and slowness (but only on direct collision). When the explosion fades, a tiny size 1 slime is revealed that rapidly path finds to slime blocks and balls over the ground and absorbs them, it increases in size by 1 level for every 50 slime collected . Players also can pick up the slime on the ground where it instantly converts to gold (5 gold for a ball, 45 for a block). When all slime has been collected, the Slime King returns to the center of the arena and loses invincibility and is set to 500HP and locked to the center of the arena at ground level. He will then rapidly shoots green particle beams at all players once per second. Upon death, the Slime King explodes dealing damage to anyone that is in a radius of 20 blocks of its explosion (being closer to the explosion will deal more damage). The main reward item is droped in the center of the arena along with a nether portal.

Slimelings*:

Slimelings instantly each choose a different player and lock onto them. They have speed and jump boost. They don’t take fall damage and each have 50HP. Attacks from slimelings give you slowness and nausea and deal damage.