Rather than saying the monsters fear the Demon King, it's more likely that the strongest in the Underground City inherently possesses some kind of "authority to absolutely command monsters."
——It's like how, in theory, Louis can also absolutely command the Curse Binding Legion.
In the later updates of the Greed Diary, the ones battling the monster army to the end were always the Curse Binding Legion.
Because it's a legion whose fear of death can be shattered by the Demon King's command, a true nemesis of living beings.
To deal with them, only members of the Curse Binding Legion who have already become Undead can manage it.
And if it's Demon King Helmer involved, then the sudden loss of contact with the Secluded Forest Sea is understandable.
This sort of instantaneous explosive power that destroys a nation-class entity, only the already formed Cosmic Serpent and Helmer can achieve.
The more he thought about it, the more reasonable it seemed.
