This page defines a group of items, Mob Souls.
Mob Souls are obtainable by killing mobs using a Hoe enchanted with Soul Stealing.
When a valid mob is killed using a Soul Stealing Hoe, the player has a 1.25*Level% chance to get the matching Mob Soul.
Mob Souls can also be found very rarely in chests:
Monster Room Chests (Zombie Souls)
Desert Pyramid Chests (Husk Souls)
Underwater Ruin Chests (Drowned Souls)
Ancient City Chests (Skeleton Souls)
Nether Fortress Chests (Wither Skeleton Souls)
Igloo Chests (Drifter Souls)
Jungle Pyramid Chests (Blooming Souls)
Mob Souls are used to set the type of Spawners. When you right click a Spawner with a Mob Soul, the Mob Soul is used up and the Spawner is set to its matching type.
A Mob Soul can also be placed somewhere other than on a Spawner. Doing this just summons the mob.
See also: Enchantments, Spawner Fragment