Wither

"New Boss: Wither! Better craft a diaper, pal."

- The Pretty Scary Update promo poster

The wither is a boss mob.

Spawning
Withers can be created by placing soul sand in a T shape (as shown in the image to the left), and putting 3 wither skeleton skulls on top of the three upper blocks. The last block placed must be one of the three wither skeleton skulls, it cannot be one of the soul sand blocks. The skull that triggers spawning can be placed by the player or by a dispenser. Air blocks are required on either side of the base soul sand block under the upper blocks. The block arrangement can have any orientation: it can be upright, flat, sideways, and even upside-down. Attempting to spawn the wither in Peaceful won't result in the structure becoming the wither. The blocks will simply stay and not move.

Like other hostile mobs, the wither does not spawn at Peaceful difficulty, and despawns when the difficulty is changed to Peaceful.

you cannot use a cheat command such as /give @a spawn_egg 64 52.

Drops
The wither drops 1 nether star upon death which, until picked up, will stay on the ground and never despawn. The Looting enchantment does not work on the wither, meaning only 1 nether star is dropped by the wither. It also drops 50 experience when killed by a player or tamed wolf.

Behavior
When spawned, the wither will temporarily flash blue and grow larger in size as its health bar fills from half to full. While in this state, the wither is invulnerable, and will not move or attack. When this state ends, the wither will create a very large explosion centered on itself, destroying nearby blocks. This explosion does not damage the wither itself, but it will damage nearby mobs/players. A ghastly sound is played when the explosion occurs. This sound is not centered around the wither, instead it can be heard equally from all areas of the map, and can also be heard by other players throughout dimensions. After this, the wither can be attacked and can also act freely.

The wither is immune to fire, lava and drowning damage. Suffocation is ineffective, since upon taking damage, the wither will break any block that comes into contact with it (except bedrock, end portal blocks, end portal frames, command blocks, and barriers).

The wither is hostile to the player and all mobs except undead mobs (skeletons, zombies, zombie pigmen, wither skeletons, husks, strays and other withers). Upon noticing a mob, the wither attacks them with a projectile called the “wither skull”. Each head fires independently, allowing a wither to attack up to three different mobs/players at the same time.

The wither has a dark purple boss health bar which is only shown when a player is looking in its direction, and can also be seen through walls. When looking at the wither, the sky gets slightly darker. All withers will display a boss bar on the screen. In the console edition, only one boss bar will be shown, and it will be a pink color (similar to the ender dragon's bar).

If the wither is calm, it simply hovers and is unable to jump. On normal or hard, it will also randomly fire blue skulls from one of its smaller heads in random directions at an interval of 2-3 seconds until it finds a target.

When its health drops below half, the wither will gain the "wither armor" effect, which makes it immune to arrows. While in this state, the wither will only fly at the same height as its target. If it regenerates above half health, the armor will disappear, and the wither is able to fly high again.

Like other undead mobs, the wither is harmed by healing potions and healed by harming potions; however, it is unaffected by any other status effect. It also regenerates every 20 ticks (1 second). The wither is unaffected by all attacks from undead mobs except its own wither skulls and other withers' wither skulls.

In console versions, the Wither can still see players when they are covered by a Potion of Invisibility and not wearing armour.

Pocket Edition
In Pocket Edition, the wither has a different behavior. At half health, it spawns 3-4 Wither Skeleton s(except on Easy Difficulty) and causes an explosion. The wither will not regenerate past half health again, and, in this state, will do a "dash attack." It will make its body flat and dash towards its target like a missile, destroying most blocks in its way. It also has new birth and death animations, both of which involve the wither spinning and exploding.

Wither skull
The wither shoots wither skulls, which look like extra heads of itself launched as projectiles, leaving a trail of smoke, and can keep up a fairly constant barrage of them. Wither skulls explode with the same force as a ghast fireball. If a wither skull hits a mob/player on Normal or Hard difficulty, it inflicts 40 seconds of the Wither II effect (which drains their health and turns their hearts black), while healing the wither for health. If the player or the mob already has the wither effect, the remaining time of the effect is lengthened instead of rewritten, which means players will suffer from the wither effect for minutes after they are hit by a lot of wither skulls in succession.

There are two types of wither skulls: a fast moving black one, and a slow moving blue one. The black wither skull cannot break blocks with a blast resistance above 20, like a ghast's fireball. They are also the same speed. The blue wither skull is shot primarily on normal or hard difficulty, when the wither is idle (meaning that no players/mobs are within its aggro range). On average, 0.1% of the shots by the main head (when the wither has a target) are blue skulls as well.

Although the blue wither skull has the same explosion strength as the black skull and are much slower (33% the speed of black skulls), it treats all blocks (except bedrock, end portals, end portal frames and barriers) as having a blast resistance of no more than 4, making it much more destructive to the terrain, and allowing the wither to break out of almost all forms of containment. Unless the blue wither skull was shot by the main head during its attack on a target (~0.1% chance), the blue wither skull would be sent in a random direction.

Wither skulls are also entities which have associated entity data. Their entity ID is.

Data values
Withers have entity data associated with them that contain various properties of the mob. Their entity ID is.

Trivia

 * There is a painting showing the soul sand and wither skeleton skull formation needed to spawn a wither.
 * The wither can see players/mobs under the invisibility potion effect.
 * The wither does not take damage from undead mobs' attacks or from drowning.
 * If the wither is damaged by the Void, it will frantically attempt to fire blue skulls.
 * If the wither is after a blaze and the blaze tries to fly up to it, the wither will attempt to keep rising above the blaze, thus making both of them rise higher until they hit a ceiling or until the blaze despawns.
 * Although the wither is 3.5 blocks tall, 3 blocks wide, and 1 block thick, its hitbox is only 3.5 blocks tall, 1 block wide, and 1 block thick.
 * The true dimensions of the wither are (confirmed by this website) 2.844 blocks tall (varies), 3 blocks wide and 1.719 blocks long (also varies).
 * The wither appears to not shoot blue skulls on the easy difficulty. It also can't apply the wither effect to attacked entities.
 * A variant of the wither called the friendly wither was in two of the April fools updates.
 * The wither spawn structure can be built any way, such as downward or horizontal, however the wither will spawn at the foot of the structure.
 * The wither appears on four sides of chiseled red sandstone.
 * Withers have the greatest natural health of all vanilla mobs in Minecraft, making it brawnier than even the ender dragon. The wither, however, is slower and deals less damage than the ender dragon.
 * The wither's smaller heads shoot targets slightly slower than the main head. the interval for smaller heads is 2-3 seconds, while the main head's always 2.
 * An enemy called the Wither Storm appears in Minecraft: Story Mode. It was created by replacing the center block of soul sand with a Command Block.