The One Who Rules Over the Netherworld

The end of terminus enters its final days. Iruma safely finishes his homework and is invited by Sullivan to go to Magical Street along with Opera. They browse around picking up things Iruma will need for his new school term. Iruma happens to overhear some demons talking and then asks Sullivan about the Demon King, since he is one of the most qualified individuals to become Demon King. What exactly does it mean to be the Demon King, the most powerful being in the netherworld?