Wither

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

Spawning
Withers can only be spawned with mods, hacks, or edited Monster Spawners.

Behavior
Upon noticing a player or passive mob, the Wither attacks them with a projectile called the Wither Skull that looks like one of its heads. Wither Skulls inflict the Wither potion effect. When a Wither is spawned, a "Boss Health" bar appears.

Withers have the unique ability to "eat" blocks. When near a block that could be destroyed by one of its projectiles, it will produce an audible crunching sound. It appears to only do this when obstructed.

Withers have a strange property that when the wither is hovering it is unable to 'jump' up a 1 block gap although it can hover up stairs and slabs.

Wither Skull
Withers shoot Wither Skulls, projectiles that look like one of the Wither's heads. The Wither will fire Wither Skulls at all passive, tame, or player mobs. Upon hitting a mob, it inflicts the Wither potion effect, draining the mob's health and healing the Wither.

Trivia

 * The Wither does not spawn in 12w34a 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.
 * 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.
 * Similarly to Zombies, the Wither will lose health when hit by a Healing Potion and will gain health when hit by a Potion of Damage.
 * Potions of invisibility do not prevent the Wither from noticing mobs.
 * Zombie Pigmen shot by a Wither skull will become aggressive against the Wither and may kill it.
 * The Wither will hurt itself with its own explosions if it targets a block within explosion range of itself.
 * 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.
 * Enderman can sometimes kill Withers

Gallery
Wither Wither Wither Wither