Wither Skeleton

Wither skeletons are dangerous hostile mobs that spawn in Nether fortresses.

Spawning
Wither skeletons spawn in Nether fortresses at a light level of 7 or less. They spawn in place of a regular skeleton 80% of the time.

Wither skeletons are 2.4 blocks tall and 0.7 blocks wide.On Halloween, wither skeletons have a 22.5% chance of spawning wearing a pumpkin and a 2.5% chance of spawning wearing a jack o'lantern.

Spawn eggs
Skeleton spawn eggs have an 80% chance of spawning a wither skeleton when used in the nether. Spider spawn eggs have a 0.8% chance of spawning a spider jockey with a wither skeleton.

In the Console Edition, all skeleton spawn eggs spawn wither skeletons when used in the nether.

Drops
Wither skeletons drop 0-1 coal and 0-2 bones. They have a 2.5% chance of dropping their skull when killed by the player (or tamed wolf), 3.5% with Looting I, 4.5% with Looting II, and 5.5% with Looting III. If the wither skeleton is killed by a charged creeper's explosion, they will always drop their skull (provided that the two mobs encounter each other). They may also rarely drop their stone sword, which will usually be badly damaged, and always unenchanted. They drop 5 experience when killed by a player or tamed wolf.

Halloween
If a wither skeleton wearing a pumpkin or jack o'lantern is killed using a tool enchanted with Looting, there will be a chance equivalent to the level of Looting used to drop the pumpkin or jack o'lantern, up to a maximum of a 3% chance of a drop.

Behavior
Wither skeletons walk slowly when idle, but will sprint towards the player when they see one. They attack with a stone sword. When the player is attacked by a wither skeleton, they are inflicted with the Wither I effect for 10 seconds, which turns the health bar black and decreases the health by every second. Wither skeletons deal on Easy,  on Normal and  on Hard.

Like other undead mobs, wither skeletons may spawn with the ability to pick up dropped melee weapons and armor.

Immune
Wither Skeletons are immune to fire, lava, and the sun

Data values
Wither skeletons have entity data associated with them that contain various properties of the mob. Their entity ID is.

Trivia

 * If hit by a wither's skull, wither skeletons can be afflicted with the Wither effect.
 * When a spider is spawned in the Nether (by the player, monster spawner, etc.), it has a 0.8% chance of spawning as a Wither Jockey.
 * Wither skeletons have the same ID as normal skeletons, and before version 1.9 they would attempt to hide under trees in daytime in the Overworld, regardless of the fact they cannot burn. This has been removed in 1.9, but they still set on fire in the daytime then immediately get put out. This was the case with Zombie Pigmen for a while, but was fixed recently.
 * Although they may spawn with the ability to pick up items, wither skeletons will never pick up a bow under any circumstances.
 * If a wither skeleton is wielding a bow (through the use of commands), it will use it like an ordinary skeleton, but the arrows it shoots will always be set on fire, regardless of enchantments.
 * The command to summon a Wither Skeleton with a stone sword is /summon Skeleton ~ ~ ~ {SkeletonType:1,HandItems:[1:{id:stone_sword}]}.
 * You can also summon one with a bow: /summon Skeleton ~ ~ ~ {SkeletonType:1,HandItems:[{id:bow}]}.

Gallery
Witherskelett Esqueleto Wither Wither squelette Scheletro Wither ウィザースケルトン 위더 스켈레톤 Witherskelet Mroczny szkielet Скелет-иссушитель 凋灵骷髅