The wither is a new potential boss mob that was added in the 34th weekly snapshot but with it's spawn disabled and can only be spawned in with the use of a mod or MCedit. It kills all passive mobs including wolfs, ocelots, villagers, iron, and snow golems except squid and will ignore you even in survival if there are passive mobs and/or villagers. So where will it spawn? Will it be a underground boss or a nether boss? Tell me what you think. I will list all of it's abilities below if you want to know every thing about it.

• As the Wither's attributes and functions are not finalized, staff such as Dinnerbone do not acknowledge the Wither's existence.
  
• 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, which is even higher 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.
  
• Withers can be damaged by the explosions of their own projectiles, should they land within range of it.
  
• 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 from the Wither effect has its own death message: [playername] withered away.
  
• Withers are classified as "undead" and therefore can receive extra damage by a sword with the "Smite" enchantment.
  
• The Wither can be damaged by snowball.
  
• When provoked by a skeleton or another Wither, either or both will only remain hostile toward each other for a moment before returning to friendly status.
  
• They can move exceptionally fast, so no mob or player can be fast enough to outrun them.
  
• 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 the player on the other so as it must path through the wall to attack. 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, although making them unable to fly.
  
• 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.
  

(I got this info off of minecraft wiki and thought I should share it because not many people new a lot about the wither)