Impudence.

The demon kings and demon lords were surprised by the need for secrecy about the deal that the realm lord offered. An oath is not a small thing to make. Breaking it will lead to dire consequences. So some of them are not willing to subject themselves to one. Especially when they don't know what exactly they are going to bind themselves to. So the demon God's assurance of safety filled them with relief. That relief turned into fright when they heard the demon God's next words.

"You have nothing to worry about if you leave. The only thing you might have to worry about is your safety after this meeting. The coalition of the abyss might need to consolidate the entire strength of the abyss and unify our forces and resources. You might encounter unfortunate events during the process. It will be very unfortunate. But it can be avoided if you swear loyalty now."