Wither skeletons in the game code are actually regularYou can summon wither skeletons marked with additional 'flag'. That 'flag' is checked in the code to change their size, weapon and drops. That flag is initialized when the skeleton is created - it is always 'regular skeleton' if the skeleton is created in the Overworld; and it is 'wither skeleton' 80% of the time if the skeleton is created in the Nether.following command:
/summon Skeleton ~ ~ ~ {SkeletonType:1}
So, there is no way to spawn witherWither skeleton inspawner can be created at the Overworld in vanilla Minecraft; not evenblock where your feet are with commands.this command:
But you can always spawn a wither skeleton in the Nether and push in through a portal to the Overworld.
/setblock ~ ~ ~ mob_spawner 0 replace {EntityId:Skeleton,SpawnData:{SkeletonType:1}}