Wither Skeleton

Wither Skeletons are dangerous hostile mobs that spawn in Nether Fortresses. Wither Skeletons are the Nether equivalent of skeletons. Unlike skeletons, however, Wither Skeletons use stone swords and are over two blocks tall. When hit by a Wither Skeleton on Normal or Hard difficulty, the player is inflicted with the "Wither" effect for 10 seconds, darkening the health bar and further damaging the player. When killed, the mob may drop coal and bones, and very rarely drop their skull and the stone sword. The skulls are the key items to creating the Wither boss.

Wither Skeletons, like all Nether mobs, are immune to fire and lava and are not damaged by daylight. Some Wither Skeletons can also pick up items, weapons, tools, and armor, similar to other undead-type monsters.

Spawning
Wither skeletons only naturally spawn in nether fortresses. Block type does not matter with naturally spawned Withers (e.g., replacing a Nether Fortress' bricks with normal blocks will still allow wither skeletons to spawn); they follow the same spawning conditions as naturally spawning Skeletons: light level 7 or less where their lower half will be (needs confirmation).

When used in the Nether, Skeleton spawn eggs are able to spawn Wither Skeletons instead of the normal skeletons 80% of the time. Similarly, when a spider is spawned in the Nether (whether via spawn egg or a mob spawner), it has a slight chance of spawning with a wither skeleton jockey instead of an ordinary skeleton.

Combat
Wither Skeletons are tough mobs to fight due to their high attack strength, and because they inflict Wither, as well as they are likely to coexist with a Blaze.

If there are lots of Wither Skeletons and Blazes around, bring some Blocks of Iron and a pumpkin to make an Iron Golem. This is also a good way to farm the drops of the Wither Skeleton.

If an Iron Golem is not an option it is advised that the player treat the Wither skeleton as they would a creeper, albeit with far more care. If you attack and move backwards at the same time you should not have any trouble avoiding damage provided you have some room to maneuver. If you have your back to a wall spam attacking should be enough to kill a single Wither Skeleton without taking damage. If you have your back to a wall and are facing more than one Wither Skeleton it is strongly advised that you move to a position where you have room to maneuver as the Wither Skeletons move slightly faster than zombies and so more easily overwhelm you. Sniping with a bow is also effective although it is not recommended at under 10 blocks away (without a knockback enchantment) as Wither Skeletons move surprisingly fast.

If you find yourself facing a Blaze at the same time the best course of action is to move into a place inaccessible for the blaze -such as a corridor- and then proceed in taking the above steps to kill the Wither Skeleton. If this is not possible it is suggested that the player run out of range of the blaze and turn to face the Wither skeleton. It is strongly advised that inexperienced players do not face both mobs at the same time as this often leads to death, but for more experienced players willing to face both you should either; deal with the blaze first by charging at it with a sword (do not use a bow as you will be exposed to attack) and then turning and facing the Wither Skeleton or (more recommended) putting the Wither Skeleton between yourself and the Blaze so as to use its body as a shield against the Blaze's fireball attack, and then charging the Blaze once it is dead.

Like all mobs, Wither Skeletons can travel through a Nether Portal. This means that if you put a Nether Portal inside the Nether Fortress, and yourself between the portal and a Wither Skeleton (or any other hostile mob), the Wither Skeleton will travel towards you but teleport into the overworld. This means that once you get back from the Nether you might still be up for a fight against a Wither Skeleton.

It is also possible to use an invisibility potion therefore be able to fight the Wither skeleton with only stone sword and no armor. This is one of the easier ways to kill a wither Skeleton but it requires you have a Invisibility potion.

Bugs

 * Because of their shared entity ID with skeletons, their behavior is similar, such as their hitboxes are the same as skeletons, along with other characteristics like heading away from sunlight despite that they cannot catch fire. Also, the Wither Spider Jockey is a glitch, because of their behavior, and only exist in Creative mode through spawn eggs.
 * In Easy, some Wither Skeletons will still inflict the Withering Effect. The rate of this occurring is approximately 62%.

Trivia

 * Their drops would suggest there is coal rubbed on their bones or that they are charred, however they are immune to fire. Also, as regular Skeletons spawn in the Overworld more frequently than Wither Skeletons in the Nether, it may be possible that they are just unique.
 * The drop rate for the wither skeleton skull is 1/40 (2.5%), as with other rare drops.
 * A normal skeleton will spawn in a Nether Fortress instead of a Wither Skeleton 20% of the time.
 * Wither Skeletons wield stone swords, yet there is no natural stone in the Nether (or any other weapon-building material, for that matter, which also would put the Zombie Pigmen's swords in question. It is possible that weapons may have gone through Nether Portals at an earlier time.)
 * Despite being larger than regular skeletons, wither skeletons aren't more resistant.
 * Their drop of coal makes it possible to craft fire charges without leaving the Nether, a way to reignite a portal.
 * This also means that coal has become a renewable resource.
 * Wither skeletons could be seen as to skeletons as cave spiders are to regular spiders, because they both are a mob-variant and have a special player-damaging after-effect
 * There is a common misconception that the Wither effect gives Wither Skeletons health back when it actually only damages the player.

Gallery
Witherskelett Wither squelette Czarny szkielet Скелет-иссушитель 凋灵骷髅