How is this fair?

The dark energy that had gathered took the form of a demon, but…that demon appeared right above The Penetrator.

No, it wasn't even above The Penetrator.

The dark energy had gathered around The Penetrator and eventually created that demon figure around it, so The Penetrator was actually already inside the demon when it formed.

So The Penetrator was actually deep inside the demon king's butt when he appeared.

That was why the demon king had suddenly screamed like this.

The demon king could only writhe in pain as smoke started to come out of his butt.

It wasn't just that The Penetrator had entered into the butt of the demon king, it was also that there was the effect of the holy water that had been sprayed on it.