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 while giving the Wither Skeleton a small health boost. 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. Brick type does not matter with naturally spawned Withers (e.g., replacing a Nether Fortress' bricks with normal stone bricks will still allow wither skeletons to spawn); they follow the same spawning conditions as naturally spawning Blazes.

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. The best way to fight them is to use a bow and shoot three arrows at them (two if the bow has Power I or higher). If the player is being attacked by several Wither Skeletons, then it would be wise to find an area that's two blocks high (Wither Skeletons can't fit in an area less than three blocks high).

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.

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 occuring is estimated to be around 5/8, with 5 of 8 inflicting the Withering Effect.

Trivia

 * Their drops would suggest that they are black because 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. It is unknown whether this is intended or merely a bug.
 * It is worth noting that the Wither Skeleton seems to not have its own Entity ID, instead sharing one with normal Skeletons.
 * That's why wither skeletons have the same hitbox as their regular counterparts despite being taller and why they attempt to hide from sunlight despite being fireproof.
 * 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.

Gallery
Witherskelett Wither squelette Czarny szkielet Wither скелет 凋灵骷髅