Wither

Withers are a new mob added in 12w34a, but left unfinished by Jeb and hidden in the code. They were discovered through the presence of their new skin in the /mobs/ folder. They are floating, three-headed skeletal creatures.

Spawning
Withers can only be spawned with edited Monster Spawners, mods, or hacks. Also, they can not be spawned with the Mob Spawner GUI.

Behavior
Withers are automatically hostile to the player and all passive mobs except Squid. They are also hostile to both utility mobs, Snow and Iron Golems, and both tameable mobs, Ocelots and Wolves. They are friendly to all neutral and hostile mobs unless provoked.

Upon noticing a mob it is normally hostile toward, the Wither attacks them with a projectile called the Wither Skull. Wither Skulls inflict the Wither potion effect. This effect is similar to poison, except that it can actually kill the player and turn their health bar black. Each head fires individually, allowing a Wither to attack up to three different mobs at the same time.

Withers have the unique ability to "eat" blocks. When near any block that could be destroyed by a player (except Bedrock), they will produce an audible crunching sound. This sound is the same one used for zombies breaking down doors. This event can be triggered by having the Wither on one side of the wall and you on the other. When you throw a potion of healing, it will harm the Wither and the Wither will "eat" a 3x3 hole in the wall.

Unlike the Ender Dragon, 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, a Wither will gain Wither armor effect, which makes them immune to arrows as well as having a visible effect surrounding them similar to a Charged Creeper. The Wither, when calm, cannot fly; it simply hovers above the ground and cannot "jump" up a one-block gap. When angered by the player, the Wither will fly at the same height as the player. Like Zombies, Skeletons, and Zombie Pigmen, Withers are harmed by Potions of Healing and healed by Potions of Harming.

Wither Skull
Withers shoot Wither Skulls, which are projectiles. Upon hitting a mob, it inflicts the Wither potion effect, draining the mob's health and healing the Wither. Note: The Wither will only heal off the player if their hunger is above nine. Similarly to Ghast Fireballs, this projectile can not break cobblestone or moss stone blocks, although it can break products of them such as slabs and walls.

Bugs

 * Withers are not affected by the lighting of the area, always being fully lit.

Trivia

 * 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 ID of the Wither Mob is "WitherBoss". This suggests that the Wither may be a planned boss mob, similar to the Ender Dragon - the idea is supported by the Wither's abnormally high HP meter, higher even than that of the Enderdragon and the highest of any mob "added" to minecraft so far.
 * To repair the entity.WitherBoss.name bug when getting killed by a Wither, add entity.WitherBoss.name=Wither to the *.lang file, after the line entity.EnderDragon.name=EnderDragon.
 * Potions of Invisibility do not prevent the Wither from noticing mobs.
 * The Wither will hurt itself with its own explosions if it targets a block within explosion range of itself.
 * Withers seem to share the same AI attribute as Skeletons, whereas if shot by a Skeleton or another Wither, they will only be hostile toward each other for a moment before turning passive again.
 * When a Wither shoots another Wither, the second will fire back a single skull and turn neutral again.
 * The verb to wither means to shrivel up and die. This is a reference to its attack which "withers" the victim.
 * Withers are unaffected by fire and lava (like nether mobs), though, like most mobs, it can drown.
 * The command /give 383 64 will give the desired amount of mob eggs set to spawn a Wither. However, nothing happens when the player attempts to use it.
 * Dying by a Wither has its own death message: [playername] withered away.