Wither

The Wither is a floating, three-headed, ghostly creature with a skeletal appearance. It is the second boss mob introduced, after the Ender Dragon.

Spawning
Withers can now be spawned since snapshot 12w36a

It is possible to spawn a Wither by placing Soul Sand in a T shape, similar to that of the iron blocks on the iron golem, and putting 3 wither skeleton heads on top

When spawned, the Wither will flash blue and grow larger in size as its health bar fills up. During this state, the Wither is immune to all damage until its health maxes out.

Behavior
The Wither is hostile to the player and all passive mobs except Squid (this is probably because it is classified as a "water mob" in the code). It is also hostile to both utility mobs (Snow and Iron Golems) both tameable mobs (Ocelots/Cats and Wolves). It is also aggressive to all other neutral and hostile mobs except for Skeletons, Zombies, Zombie Pigmen, Wither Skeletons and other Withers. 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.

The Wither can move very quickly. Upon noticing a mob that it is normally hostile toward, the Wither attacks them with a projectile called the Wither Skull. Wither Skulls inflict a status effect called "withering". This effect is similar to poison, except that it can actually kill the player and turns the health bar black, making it difficult to tell at a glance how many hearts are still full. Each head fires individually, allowing a Wither to attack up to three different mobs at the same time.

The Wither has a unique ability to destroy blocks. When near any block that can be destroyed by a player, it will produce an audible smashing 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, for example, a wall, and the player on the other, so it must path through the wall to attack. The Wither will then destroy a 3x3 hole in the wall to pass through it.

Unlike the Ender Dragon, the Wither's boss health bar is only shown when a player is looking in a direction of a Wither, at least in the current implementation, and it can also be seen through walls when the player is looking in the Wither's direction. When reaching half health, the Wither will gain the "Wither Armor" effect, which makes it immune to arrows, as well as having a visual effect surrounding it similar to that of a Charged Creeper. However, this effect renders it 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 other undead mobs (Zombies, Skeletons, etc.), the Wither is harmed by Potions of Healing and healed by Potions of Harming.

Wither Skull
The Wither shoots "Wither Skulls", which are its extra heads launched as projectiles. These heads grow back very quickly, so it can keep up a fairly constant barrage of them. Upon hitting something, a Wither Skull will explode, although the blast radius is much smaller than that of TNT or a Creeper. If the explosion damages a mob or a player, it inflicts the withering effect, draining their health and healing the Wither. Note: The Wither will only gain health from the player if their hunger is above nine. Similar to Ghast fireballs, this projectile can not break blocks with a Blast Resistance above 20.

Bugs

 * The Wither is not visually affected by the lighting level of the area, always being fully lit.
 * The Wither attacks Magma Cubes without any reason.

Trivia

 * 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", and it has a visible health bar, like the only other boss, the Ender Dragon. This confirms that the Wither is planned to be a boss.
 * Potions of Invisibility do not prevent the Wither from noticing mobs.
 * The Wither can be damaged by the explosions of its own projectiles.
 * Dying from the Wither effect has its own death message: [playername] withered away.
 * Withers are unaffected by fire and lava (like Nether mobs), and do not 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.
 * The Wither is classified as "undead" and therefore receives extra damage from a sword with the "Smite" enchantment.
 * It is possible to just walk forward calmly and the Wither's skulls won't hit the player, as it is constantly adapting its speed to its target.
 * The Wither Shield glows red when a wither with the shield gets hit.
 * The sky will turn slightly reddish when you look at it.