Wither Skeletons are dangerous hostile mobs that spawn in Nether Fortresses. They are the Nether equivalent of skeletons. Wither Skeletons are three blocks tall, use stone swords and, like other Nether mobs, are immune to fire. When attacked by a wither skeleton, the player is inflicted with the Wither effect for 10 seconds, which darkens the health bar and gradually damages the player over time. When killed, wither skeletons can drop coal and bones, and have a 2.5% (1/40th) chance to drop their skull or a stone sword (the game doesn't decide when it dies to drop something and randomly choose between the 2, rather, it will has a 1/40 chance to drop each, or a 1/1600 chance to drop both, assuming the chance is 1/40 for the sword, but it is more likely than that). Wither skeletons can spawn with tools, weapons, and/or armor. The wither effect acts just like poison, with a very important exception: it can kill you, but poison cannot kill you no matter what.
Wither skeletons are three blocks tall, and have dark, gray bones. Wither skeletons appear to have long streaks of black marks across their face.
Wither skeletons will only spawn near Nether fortress in light levels of 7 or less. Skeleton spawn eggs are able to spawn wither skeletons instead of regular skeletons 80% of the time in the Nether.
Wither skeletons are tough to fight due to their high damage and their ability to sprint. Their fast movement usually yields them at least one hit on the player each encounter. Because you will often have to fight them while fighting blazes, it is often difficult to avoid being hit by one mob or the other. Their biggest weakness is that they cannot pass through a two block high passage because of their height, much like endermen.
If you attack and move backwards at the same time you should not have any trouble avoiding damage provided you have some room to maneuver. If you have your back to a wall spam attacking should be enough to kill a single wither skeleton without taking damage. If you have your back to a wall and are facing more than one wither skeleton it is strongly advised that you move to a position where you have room to maneuver as the wither skeletons move slightly faster than zombies and so more easily overwhelm you. Using a bow is also effective although it is not recommended at under 10 blocks away as wither skeletons move surprisingly fast. If you find yourself in a position where a wither skeleton is a one shot, use a diamond sword or axe with high level Smite/Sharpness and ideally, Looting enchantments.
If you find yourself facing a blaze at the same time the best course of action is to move into a place inaccessible for the Blaze (such as a corridor) and then proceed in taking the above steps to kill the Wither Skeleton. If this is not possible it is suggested that the player run out of range of the blaze and turn to face the Wither Skeleton. It is strongly advised that inexperienced players do not face both mobs at the same time as this often leads to death, but for more experienced players willing to face both you should either; deal with the Blaze first by charging at it with a sword (do not use a bow as you will be exposed to attack) and then turning and facing the Wither Skeleton or (more recommended) putting the Wither skeleton between yourself and the Blaze so as to use its body as a shield against the Blaze's fireball attack, and then charging the blaze once it is dead.
You can also build partial walls in a nether fortress that fall down from the ceiling and leave a walkway two blocks high. Wither skeletons can't move through two block high spaces so building such structures limits their mobility, and will allow you to damage them with impunity.
If a wither skeleton is able to acquire a bow (currently by natural spawn only), it will use it to shoot at you. Any arrows it shoots will be on fire, and will cause you to catch fire, regardless of the bow's enchantments. If a Wither Skeleton picks up a sword enchanted with Fire Aspect, it becomes a considerable threat as the combination of the fire damage and the Wither effect can quickly prove fatal, especially if the sword is made of iron or diamond.
Fighting with Wither Skeletons on exposed fortress walls is good for getting rid of them with minimal effort (particularly with a sword that has Knockback). This comes at the cost of losing the drops, however, and players need to be wary of Ghasts as well. But if you are looking for the rare drop "wither head" its not good and you may lose the chance to get the wither head.
Another viable tactic is more defensive than offensive. Place "bars" at every intersection (and occasionally in long corridors, if desired) that are tall enough for the player to pass under, but not Wither Skeletons. This works well to prevent ambush, but one must still be aware of their surroundings, especially if a Blaze is nearby.
Be warned that Wither Skeleton attacks carry Knockback, and it is entirely possible for them to knock you out of a Nether Fortress, especially if you time a jump badly. This may lead to death by fall or death by lava.