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, the player may be inflicted with the "Wither" effect for a few 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. Wither Skeletons can also occasionally pick up items, weapons, and tools, or spawn naturally with armor, similar to other undead-type monsters.

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.

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 Nether Fortresses more frequently than Wither Skeletons, 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.
 * Brick type does not matter with naturally spawned Withers i.e., replacing a Nether Fortress's bricks to normal stone bricks will still cause wither skeletons to spawn; they seem to follow the same spawning conditions as naturally spawning Blazes.
 * 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).
 * Despite being larger than regular skeletons, wither skeletons aren't more resistant.

Gallery
Witherskelett Wither squelette Wither скелет 凋灵骷髅