Mortum Souls are the mob drops obtained from killing most mobs that are native of the Mortum Depths. The main use of Mortum Souls is the crafting of several Boss Spawner Crystals. Mortum Souls can also be placed in a crafting sqaure to produce Mortum Fragments, which is the best method to farm Mortum Fragments.
How They Are Obtained
Mortum Souls can be obtained by doing any of the following:
Base Spawn Crystal, Dravite, Azurite, Mortum Souls
Base Spawn Crystal, Azurite, Uvite, Mythril, Mortum Souls
Base Spawn Crystal, Azurite, Mythril, Mortum Souls
Twilight Demon Crystal
Base Spawn Crystal, Uvite, Mythril, Mortum Souls
Base Spawn Crystal, Mortum Souls
Though it is not clear whether or not this is a glitch, an Uvite Archer (native to the Uvite Islands) will drop Mortum Souls upon death, while a Twilight Archer (a native of the Mortum Depths) will drop Uvite Souls upon death.