The Demonic Spirit is an unnamed, mysterious, malicious, and extremely powerful being who devours souls.
The origins of this creature are unknown, but it has seemingly devoured countless victims, the latest being Kuni's family. After capturing them, the monster kept her alive and forced her to lure in more victims for it to devour. She was crying in the woods, when Jack met her she ran back in the house causing Jack to follow.
After Jack investigates the house, the demon finally reveals itself, but despite Jack's best efforts, the demon gains the upper hand and consumes Jack's soul. An intense battle ensues inside the demon's own realm, which Jack eventually wins, destroying the monster by deflecting it's own fireball attack at it and finally freeing the souls of it's victims.
When Jack returns, Kuni is reunited with her family, as the demon's curse on her house has been lifted.
It is unknown how the past Aku's destruction affected his existence.
The demonic spirit can possess and curse areas and structures, such as Kuni's home, once under it's control, it can manipulate reality to trap victims who enter.. In it's true beastly form, the demon can create dark flaming weapons such as a hammer or sword for melee attack and a dark fireball for range attack. It is revealed that he can control the trajectory of the fireball like a homing missile toward its target. However, the spirit can be destroyed by its own attack as Jack kills the spirit by deflecting one of his fireball back at him.
- This creature is one of the few antagonists that has no affiliation to Aku in any way. It can also be considered one of the most malicious and terrifying enemies Jack has ever faced.