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 spawned with mods, hacks, or Monster Spawners.

Behaviour
Upon noticing the player the Wither fires a projectile known as the Wither Skull towards them which appears similar to its heads. They will also target passive mobs. If the Wither Skull hits the player, they gain a "Wither" potion effect. This effect drains health, similar to poison potions. If you spawn a Wither, 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.

Gallery
Wither Wither Wither Wither