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For the Moment, souls are only dropped from players killing the mob with fortune.
Currently Availble mobs.
Skeleton
Zombies
Spiders
Cave Spiders
Blazes
There are plans to add them to more loot tables later down the track.
Souls are an item type used to convert Spawners
Souls are used in converting Mob Spawners into the corresponding mob type when right-clicked (ie Soul Spider can convert a skeleton spawner into a spider spawner)