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 appear to be floating three-headed skeletal creatures.

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

Behaviour
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. If the Wither Skull hits a player, they gain a "Wither" potion effect, tinting the player's health bar black, draining health like poison potions, and healing the Wither at the same rate as the player loses health. When a Wither is spawned, a "Boss Health" bar appears. The Wither also has the ability to break multiple blocks at once.

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.
 * The noise the Wither makes when it destroys blocks is the same as the noise made when a zombie breaks down a door.
 * Pigzombies shot by a Wither skull, will not become aggresive against the Wither.
 * The Wither can hurt himself with his own explosions, when the target is right next to him.
 * When a Wither shoots another Wither, that one will fire back one skull and get neutral again.

Gallery
Wither Wither Wither Wither