The wither skeleton is a hostile mob found in nether fortresses. They appear as a black skeleton, and they spawn with a stone sword. Generally, they are very annoying to fight, because when they hit you, they give you the withering effect. The withering effect damages you overtime, and unlike the poison effect, it will kill you if you get too low health while it is active. Even though they are so annoying, they are very useful, as they drop wither skeleton skulls, which are used to summon the wither (A boss that drops an item essential to crafting a beacon).