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 as a jockey for a wither skeleton instead of an ordinary skeleton.

Trivia
-Their drops would suggest that they are black because there is coal rubbed on their bones, however they are immune to fire.

Gallery
Witherskelett Wither squelette Wither скелет 凋灵骷髅