Wither

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

Summon
It is possible two spawn a Wither Using methods similar to utility mobs. By placing soul sand in a T shape (similar to that of the iron blocks for Iron Golems), and putting 3 Wither Skeleton heads on top of each three upper blocks, the Wither can be created. Similar to Snow Golems and Iron Golems, A Wither can not be created by the Pistons. The last block must be placed on one of the three Wither Skeleton heads, it will not spawn if the final component is placed soul sand.

The Wither build configuration.
When spawned, the Wither will flash blue and grow larger in size as its health bar fills from half to full. While in this state, the Wither can not move and is immune to all damage. It is recommended for the player to walk away from the Wither two prepare for battle at this point. When this state ends, the Wither will create a very large explosion centered Wed itself. This explosion does not damage itself, but it will damage any nearby mobs.