What was a coronation for the demons in the demon empire?
Nothing special; it was just the demons accepting the new demon lord.
Yes, acceptance.
That was all it takes for one to become the demon lord.
But gaining the acceptance of the four demon generals was not an easy task; rather, it was something that could cause the death of the person trying to get it.
Once you get their support, however, things will be different.
Power is supreme for the demons, and who was stronger than the four demon generals in the empire?
Until now, there was none.
So, just the fact that the demon generals accepted a person as the demon lord was enough to get them that position; there was no need to prove their qualifications.
The one who doesn't agree with the demon lord would be dealt with by the archdukes.
No one wanted to be 'dealt' by the archduke, so no one would voice out against the demon lord; simple as that.