Kibutsuji Muzan was—is a demon. The Progenitor of Demon, to be exact. There's no use in denying that. His unhealthy pale skin, his inhumane blood red eyes with slit pupil, cold body temperature, fangs, and his inability to walk under the sun directly. And one can't forget to mention he need to consume blood on certain schedule. Proof that he was not a human.
In another world, he also known as Cruel Demon. In that world, Muzan is obsessed to conquering the Demon's universal weakness: the Sun. In order to accomplish that, he creates many legions of demon. Transforming Human into Demon. Some become Demon by their own accord, some he tricked into become a demon. In that world, he take delights in seeing other suffers. Especially if they manage to offend him, mostly by mistake or ignorance. Human, demons. It doesn't matter. Whoever offends him shall suffer.
In this world, however...
"MUZAN-SAMA! HELP!" A terrified scream echoed throughout the Demon Headquarter. Causing Muzan to jolted in surprise. "DOMA WAS AT IT AGAIN!"
"God. Damn. It!" Growled Muzan, seeing his four-hours work of painting now was in ruin. Tearing it off, he threw the paper at trash bin at corner of the room, before stood up and stomp away.
The lesser Demons, at sight of him angrily stomp his way through, quickly moved away from his way. Some were outright bolted out of the corridor.
Muzan almost tore the shoji door when he push it aside. Scratch that. He already made a hole in it. "Alright Doma, what did you... do... this time..."
His voice become weaker and weaker as his anger subsided, replaced by incredulity until all he can do is staring blankly at the scene before him. The reason? This!
Doma was hugging Kokushibou tightly as he turning and dancing like possessed... or perhaps a drunk idiot. The Waning Crescent Moon was squealing "I Know your Feelings, I so Understand It Now!" in creepy cheerful manner, which proceeds to terrify Kokushibou further. A loopsided grin decorates his face.
Kokushibou, upon seeing Muzan, threw him a pleading look. Save me!
Muzan felt his brain screeched to halt.
"...what the fuck is happening here?"
(Whereas in Another World Muzan is known as Ruthless Tyrant, it does not apply here.
In This world, Muzan used to be decent human being. Before he was Kibutsuji Muzan the Demon, he had been Kibutsuji Muzan, a father to his children. A hardship called parenthood had tempered his pride into manageable level, as well giving him patience and understanding of other. It carries through even though he had been a Demon for roughly a thousand years. Just that...
...his Demons seems to hell-bent in turning him insane with their ridiculous antics.)
Silver Moon
(Kimetsu no Yaiba fic)
After a long, difficult hours in attempt to prying Kokushibou off from Doma's steel-like grip (even after Muzan forcibly call Azaka on his Days off to help), the AllFather of Demons stared at Doma with arms crossed. Kokushibou behind him, trembling like leaf with eyes blown wide in terror. He was scared that Doma will pull him into... into... into whatever that was!
His Lieutenant / Vice-in-Command had never been this terrified, not even under the harsh sunlight during that particular harsh summer.
As a precaution, Azaka stood right next to Muzan. Ready to pound Doma into the ground in case he do something.
"Alright, Spill. What did you do this time?" Muzan kneaded his forehead. He can feel migraine forming. Demon is not supposed to have migraine, yet he does. "And what did you mean by "I know your feelings, I understand it now" comment earlier?"
Why did he turn this menace into Demon again? Oh yeah. Because Doma follows him like a very annoying puppy, and proceeds to annoys the hell out of him for entire year straight!
Grinning brightly, Doma replied, "oh, I will become a Father!"
Silence. Pure, utter silence where you can hear a pin drop miles away. Every other Demons, include those who eavesdropping using their own means, was utterly petrified. Their already pale face turned even more pale.
A note here: Demon can't actually have children. Oh yes, they still be able to enjoy sex, but they were very much fertile. To increase their number, a Demon will give a human their blood, converting said human into newborn demon. That was what Procreation mean in Demon Dictionary.
In another World, Muzan is the Sole Progenitor of entire Demon Race, with exception of certain demon. Here, however, Muzan realized the potency of his blood. Anyone who receive his blood will either die a painful death, or become powerful Demon. Example 1: his Moon Phase Demons. Example 2: Abominations. Regarding the later, after creating what he dub his Biggest Mistake, Muzan had become very picky and careful in whom he turned into Demon. In his stead, his Moon Phase Demons will increase the numbers.
Except Doma.
After the first time Doma turn a human into demon, and the swiftly following consequences, Muzan had forbid him to procreate. Ever.
"...who the fuck let you procreate...?" whispered Azaka.
He still remember Doma's Spawn. Ye gads! It took them six month to clean the following mess. He don't want any repeat. One is bad enough. Two is disaster!
"Oh, piss off." Doma waved his hand. "It wasn't like that. I'm going to marry, actually!"
"...I see..."
...no one knew what they should feel. Grateful that Doma didn't have another spawn, or pitying his soon-to-be wife and stepchildren, or dreading Doma 2.0 since he will have a hand in raising his stepchild.
"So! Since Muzan-sama and Kokushibo-senpai are father do you have any tips?"
...perhaps they can mitigate any upcoming disaster?
And Akaza, the Bastard, take it as his cue to leave the vicinity. That Scum!
After painful hour of lecture of what to do and what to NEVER do, impromptu lesson on fatherhood, and some agreement to let Demons from other faction, one from Kokushibou's and one from Akaza's, to joining Doma's faction as additional helper (and to NOT torturing them to point of suicide), the Colorful-eyed Demon returning home. Humming a bright tone.
Leaving utterly (mentally) exhausted three Demons behind.
"Muzan-sama. Permission to take three-months long Holiday?"
"Granted."
Because Muzan is going to take Holiday as well. Normally either he or Kokushibo will stay at headquarter as Commander. But after this torture... he was willing to make exceptions. Dealing with Doma is a torture itself.
There's a reason why Dealing With Doma's Insanity is Number #1 Demon's Most Feared Thing, beaten Muzan's wrath and Burned down by Sun.
TBC
Aka in which Muzan is Decent person and Suffering because his Demons are like overgrown child.
