Minecraft wiki entry for Nether_survival, associated text: Wither skeletons are tall, darkened skeletons, wielding stone swords. They inflict the wither effect for 10 seconds with a hit. They walk when idle, but sprint toward a player when they see one. However, they can't pass through a two-block-high space, which makes them somewhat easy to deal with. They drop bones, coal, and occasionally their stone sword. They also have a small chance to drop a wither skeleton skull, which are essential to summoning the wither boss. These types of skeletons are found only in nether fortresses. They can roam in groups in fortress hallways, so be careful of blazes and being withered in this situation. Knockback is good for packs where you want them as far away as possible.