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 bottomless three-headed ghost-like creatures.

Spawning
Withers can only be spawned with mods, hacks, or 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 similar to their heads. Wither Skulls are fired at any passive, tameable or playable mob. Upon hitting a mob, it inflicts the Wither potion effect which drains the mob's health and gives it to the Wither itself.

Trivia

 * The reason the Wither was not made to spawn in 12w34a is 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.
 * If you want to repair the entity.WitherBoss.name bug when getting killed by Wither, simply add entity.WitherBoss.name=Wither to your *.lang file. (You should put this line after entity.EnderDragon.name=EnderDragon)
 * Similarly to Zombies, the Wither will lose health when hit with a Healing Potion and will gain health when hit by a Potion of Damage.
 * Potions of invisibility have no effect on the Wither.
 * The noise the Wither makes when it destroys blocks is the noise a zombie makes when it breaks down a door.

Gallery
Wither Wither Wither Wither