Withering Souls are an item dropped by the Wither when killed. It has a 35% chance of dropping after every kill.
The Withering Soul is used to create the Nether Star Crux. Extra souls can also be used to create Wither Skeleton Skulls.
Nether Ores
End Ores
Tier 6 Crops
Cruxes
Insanium Essence
Essence Coal
Essence Paxels
Withering Soul
Dragon Scale