Wither

The Wither is a floating, three-headed, ghostly mob with a skeletal appearance. It is the second boss mob introduced, after the Ender Dragon.

Spawning
It is possible to spawn a Wither using methods similar to utility mobs. By placing soul sand in a T shape (similar to that of the iron blocks for Iron Golems), and putting 3 Wither Skeleton heads on top of each three upper blocks, the Wither can be created. Similar to Snow Golems and Iron Golems, a Wither can't be created by pistons. The last block placed must be one of the three Wither Skeleton heads, it won't spawn if the final component placed is soul sand.

When spawned, the Wither will flash blue and grow larger in size as its health bar fills from half to full. While in this state, the Wither cannot move and is immune to all damage. It is recommended for the player to walk away from the Wither to prepare for battle at this point. When this state ends, the Wither will create a very large explosion centered on itself. This explosion doesn’t damage itself, but it will damage any nearby Withers.

Behavior
The Wither is hostile to the player and all mobs except undead mobs (skeletons, zombies, Zombie Pigmen, Wither Skeletons and other Withers). When provoked by a skeleton or another Wither, they will only remain hostile toward each other for a moment before returning to friendly status. If spawned in the End, it will prefer to attack any nearby Endermen, but will attempt to pursue and attack the Ender Dragon if there are no nearby targets.

The Wither can move very quickly. Upon noticing a mob, the Wither attacks them with a projectile called the “Wither Skull”. Wither Skulls inflict a status effect called "Wither". This effect is similar to poison, except that it can kill the player and turns the health bar black, making it difficult to tell at a glance how many hearts are still full. It also slowly heals the Wither. Note: Players can only receive the Wither effect on Normal or Hard.

Each head fires individually, allowing a Wither to attack up to three different mobs at the same time. Withers need at least 5 block high airspace to fit, but as of snapshot 12w41b, they can no longer be trapped (and suffocated) in smaller rooms. Every few seconds, the Wither will break any block that comes into contact with it, except bedrock.

The Wither has an ability to destroy blocks, similar to the Ender Dragon's ability. When near any block that can be destroyed by a player, it will produce an audible smashing sound. For example, this could happen by having the Wither on one side of a wall, and the player on the other, so it would have to make a path through the wall to attack. The Wither will then instantly destroy a 3x3 hole in the wall to pass through it. The Wither also destroys a 3x3x4 space around itself 20 game ticks (1s) after being harmed.

The Wither's boss health bar is only shown when a player is looking in a direction of a Wither, and it can also be seen through walls when the player is looking in the Wither's direction. When reaching half health, the Wither will gain the "Wither Armor" effect, which makes it immune to arrows and potions, as well as having a visual effect surrounding it, similar to that of a Charged Creeper. While in this state, the Wither cannot fly. The Wither, when calm, cannot fly; it simply hovers above the ground and cannot "jump" up one block. When angered by the player, the Wither will fly at the same height as the player. Like other undead mobs (zombies, skeletons, etc.), the Wither is harmed by Potions of Healing and healed by Potions of Harming. It also regenerates over time, even if no mob was afflicted by the "withering".

Wither Skull
The Wither shoots Wither Skulls, which look like extra heads of itself launched as projectiles and it can keep up a fairly constant barrage of them. Upon contact, a Wither Skull will explode, although with a blast radius much smaller than that of TNT or a Creeper. There are two types of Wither Skulls: a fast moving black one, the wither skull, and a slow moving blue one, called the "Block Buster". Both have an explosion power of 1, the same as Ghast fireballs; however, the blue skull has special properties. If the explosion of a Wither Skull damages a mob or a player, it inflicts the wither effect, draining their health and turning their hearts black.

The black Wither Skull has a normal blast, which cannot break blocks with a blast resistance above 20. Thus, cobblestone and other Ghast-proof blocks will resist this blast. The blue Wither Skull is shot only if the difficulty is set to normal or above, when the Wither is idle, meaning the player is not within its aggro range. Although it nominally has the same explosion strength as the black Skull, it treats all blocks (except bedrock) as having a blast resistance of no more than 4. This makes it much more destructive to the terrain, and lets the Wither blast away obsidian traps or shelters from a distance.

Combat
Because the Wither inflicts a great deal of damage, the best equipment for combat is a diamond sword (Preferably enchanted with Smite. The best sword for the fight is a Smite V, Fire Aspect II, and Knockback II) Instant Health II Splash Potions, Strength II Splash Potions, one or two enchanted gold apples, a Bow (Preferably enchanted with Power, the best bow for the fight would be Power IV (V if obtained before 1.3 or if Anvil is used), Punch II, Flame I, and Infinity I) and Diamond Armor (Blast Resistance IV resists the explosions, but Protection IV protects from the withering effect as well). Make sure to bring about a stack or two of arrows if you don't have the Infinity enchantment.

Bringing or summoning allies (Wolves, Snow Golems and Iron Golems) may or may not be advantageous: on one hand, they provide a diversion and will cause damage to the Wither: on the other hand, they may die easily because of the explosions and get affected by the Withering effect, healing the boss far more than they damage him. If you create the Wither in a small and completely enclosed underground area that isn't too large (three blocks tall), three Iron Golems are sufficient to kill one without aiding them. Spawn the Wither, and quickly run away as far as possible, putting a large distance between you and the Wither. Do not stand near the Wither when it is growing into its normal size. It will create a massive explosion when it is finished, much larger than that of TNT or a (charged) Creeper. When it is finished, walk back and shoot at the Wither, and occasionally use your Instant Health II Splash Potions and at some point, eat your Enchanted Gold Apples. When the Wither drops to 50% health, use your Strength II Splash Potion and hit the Wither with your diamond sword until the Wither is defeated. This is a very effective way to beat the Wither, defeating it in less then five minutes with not too much struggle.

An extremely easy way of beating the Wither is to dig out a 10x10 by 3 tall room and spawn 30 or so Snow Golems. The Golems act as a target for the Wither, as well as attacking it. Accompany this with your best sword (preferably with Sharpness and Fire Aspect enchantments), armor, and potions and you can beat him in under a minute taking little or no damage.

Another strategy is to obtain a diamond sword with Sharpness II and Fire Aspect I, and full iron armor with Protection I or greater. As far as potions go, get four or so Splash Potions of Healing II, and two Potions of Strength II. As long as the Wither is constantly battered from the start of the battle, stronger armor may be unnecessary.

A tactic that is relatively easy to pull off is to dig down near bedrock, and dig a long 1x2 tunnel (Longer than 30 blocks). At the end of the tunnel, construct a small room. Spawn the Wither in this room, then run back into the tunnel. Occasionally pop out and shoot an arrow or two at the Wither. As the Wither crunches its way towards you, just walk backwards through the tunnel. Once the Wither has reached its "Armor" stage, jump out and begin hitting it with a diamond sword. As long as you frequently throw down splash potions of instant healing, you should be able to survive even with iron armor.

A more advanced tactic that is very rewarding would be to construct a cobblestone generator-powered grid above the player, spawn the Wither about 15 blocks away, and stand under the grid. The Wither will fly towards the player, but it would be kept at the edge of the grid, constantly being pushed back by the cobblestone which is generating faster than what the Wither can break. This allows the player to stand still and shoot the Wither without retaliation, until the Wither is at half health, where it will fly towards the player through the grid, allowing the player to easily kill the Wither with a sword. In addition to that, the player can rig dispensers about 15 blocks away to periodically shoot splash potions of health, which heal the player and damage the wither. This tactic is explained in greater detail in this video.

It is also possible to kill the wither with a fully automatic machine, requiring no combat from the player. The most conventional design suffocates the wither with a cobblestone generator, as demonstrated in this video or this video.

Bugs
Once spawned and even killed, if the player enters another world and comes back, at least one wither will spawn despite having not been made by the player.
 * The Wither is not visually affected by the lighting level of the area, it always appears fully lit.
 * In invincibility mode, the shield is not correctly surrounding the left and right head.
 * The Wither can “crunch” through fluids.
 * If the wither is spawned in an enclosed environment, it can attempt to fly through bedrock, resulting in it being stuck in the bedrock, but it will not take suffocation damage.
 * If you logout halfway from downing him, there is a chance that once you log back in your game will repeatedly crash. This will occur even if the command (/killall) is typed or the wither is otherwise removed. Reinstalling java with the latest version can fix the region.
 * If you build the three heads of the Wither underwater, the soul sand block that is in the middle and the side that the first skull was placed on will become an unmineable, transparent block that expels entities.

Trivia

 * The Wither is 4 blocks tall, 3 blocks wide, and 1 block thick.
 * The Wither does not spawn in 12w34a and 12w34b because it can crash the Java Virtual Machine.
 * If the Wither has nothing to shoot at, it will stay still, moving very rarely.
 * The Wither can be damaged by the explosions of its own projectiles.
 * Dying from the Wither effect has its own death message: [playername] withered away.
 * Withers are unaffected by fire and lava, similar to Nether mobs. The Wither will not take drowning damage if somehow submerged in water for too long.
 * The Wither will take block suffocation damage. After the first tick of damage, it will destroy the blocks suffocating it if mobGriefing is turned on.
 * The command will give the desired amount of mob eggs set to spawn a Wither, but nothing happens when the player attempts to use it.
 * The Wither is classified as "undead" and therefore receives extra damage from a sword with the "Smite" enchantment.
 * It is possible to just walk forward constantly and the Wither's skulls won't hit the player, as it is constantly adapting its speed to its target.
 * The Wither Shield glows red when it gets hit.
 * The sky in the overworld will turn slightly reddish when you look at the Wither.
 * The Wither Skull explosions can even break obsidian. At its high blast resistance of 6000, it's a block known for its hardness and explosion resistance to TNT.
 * The only blocks the Wither cannot break are air (due to its indestructibility) and bedrock (its blast resistance being 18,000,000 and hardness being -1), probably to prevent holes in the lowest bedrock layer which would give an access to the void.
 * The Looting enchantment does not affect the Wither's Nether Star drop.
 * The Wither attacks all living mobs, including living hostiles like Creepers, Spiders, Ghasts, Endermen, and the Ender Dragon.
 * The Protection enchantment can block the Wither effect.
 * The Wither has its own painting that has instructions on how to build it.
 * Although the Ender Dragon was meant to be the final boss, The Wither is far harder to kill if it is out in the open. If it's trapped, it can easily be killed.
 * It also has 100 more health (50 hearts) than the Ender Dragon
 * Unlike Ghast Fireballs, Wither Skulls cannot be hit or shot with an arrow to be redirected. If they’re shot with arrows, they will be explode on contact, destroying the skulls and arrow.
 * It is possible to trap a Wither into a Minecart, but it will break it eventually.
 * In many foreign language translations, Wither is translated to a portmanteau of "Skeleton" and "Cerberus". This is due to the fact that it has three heads.
 * Enough Endermen can kill the Wither if the ceiling is not high enough
 * The Wither can still see you when you are using the invisibility potion and will not turn invisible when hit by an invisibility potion.
 * If you attempt to build the Wither in Peaceful difficulty, the bottom, left, and right Soulsand blocks will disappear along with the left and right Wither skeleton skulls. The middle skull will be  facing the player.