Hell is a subordinate force of the Underworld.
However, the Underworld has always had little control over these top entities like the Seventy-Two Pillars.
Especially those level 10 beings, many of whom are big players in the Underworld too.
Take the Netherworld Dog Lord, for example, it is both one of the Seventy-Two Pillars and a Nether Emperor.
Under such a complex relationship, the resulting conflicts of interest are also very chaotic.
Friction is inevitable; there are no large wars but constant skirmishes.
Most importantly, according to reliable information from the Master of Demon Fox.
The Abyss may launch a new round of Blood Battle against Hell.
This is the main reason why every member of the Seventy-Two Pillars is in a state of anxiety.
In ancient times, before being controlled by the Underworld.
Hell itself was also a hegemon, its status even higher than Nora's current position.
The most direct evidence is that half of the Seventy-Two Pillars were level 10 then.
