ShadowKing, an entity that was created by turning a magus into a host for thousands of souls and then merging them all into one. That's what the demonic sect did to Thalos.
They intentionally caused chaos to create numerous deaths. Every human has a soul, but most of their souls, especially ordinary people, would quickly erode due to mana nature and disappear. However, the higher someone's level, the higher the resistance of their soul to mana nature.
For example, apprentice magus had a 20% chance of surviving the erosion of mana nature, but 97% of the surviving souls would still experience mental disturbances, rendering them unable to think clearly at all.
