Chapter Text
//HEAD CANON THAT MUZAN KIBUTSUJI WAS ACTUALLY THE FIRST VICTIM OF THE DEMON PROGENITOR, HIS FATHER//
So it went this way:
Mr. Kibutsuji belongs to a demon worshipping clan and he is the next leader, BUT he can't ascend to that position without having an heir.
So he went marrying some random woman and he tried having a baby right away. Of course, he didn't got any. So he disposed of the woman within three months of marriage. This goes on for five years until he met someone.
Now, the Ubuyashiki clan (Kagaya Ubuyashiki-san's) is against the Kibutsuji clan about their obvious worship to Satan. Well, guess what? Muzan's mother is from that clan. And he didn't even told her about his clan, so she went and fell in love with him and ultimately for the first time in his life, he got a baby.
When Muzan was five, his father started showing his true colours. Bringing him inside the occult and preaching to him the core values of their own God: Power, and Immortality. Of course this raised an alarm, his mother started seeing red flags and her motherly instinct's are kicking in.
Fast forward, they ran away, oh they did. And his father was mad, you could even see hell freezing over because of him! Now, did I told you about the reincarnation au? No? Well, then I'm telling you right now! This is a reincarnation au!
At that same time, Muzan's mother alarmed her side of the family; the main family. Around that time, Kagaya, (yes, the current boss man of the Hashiras in the anime-cannon) is three years old. They were friends, of course more than that, because they were cousin.
Well, Kagaya-kun is worried about him. He even showed the adults that he can help by being a nosy little brat. He rallied his trained crows and ordered the feral winged animals to attack his cousin's father who's also under attack by the main family. ( He was scolded of course )
But this whole fiasco actually allowed Muzan and his mother to disappear from his father's sight and went into hiding. But really, the already strained relationship of both clans just got worst, hell, it ceased to exist.
The cat and mouse chase between the family got really long until Muzan's mid-twenty. He was captured along with the boy that they adopted from the streets with his mother. Inside his father's villa, he sat on a prison cage tortured and forced to listen to his father's preaching. Of course they still fed him, can't have the heir dying on them right?
But Muzan was cautious enough to not eat anything they serve him. And he's right! 'Cause the food that they were actually feeding him are the remains of his mother who got dragged away from him the moment they entered the villa. He knew of that, the night he decided to finally escape the hell hole and retrieve the boy that they adopted. The boy was, crying. Apologizing to him and having snots all over his tattered kimono.
Upon running away though, he was shot by a mysterious syringe by his father and strangely enough, that stopped the minions from chasing them. Now Muzan did not waste his past twenty years of life just to run away. He was also researching with the help of the Ubuyashiki clan's friend; Tamayo-san.
After resting for a bit, they both went (the boy that he adopted) to Tamayo-san's lab and told her all the that happened. She wanted both of them to stay, but was ignored. So Muzan and the boy was up and running again.
Fast forward again, the news of his mother's death and his apparent kidnapping is spreading like wildfire upon the branch family and the main family. Kagaya-kun, is now the heir of course, so he went to the mansion that he gave his cousin and visited the people that Muzan previously helped (adopted), he confirmed their confusion and that is when all hell broke loose.
Muzan was delivered a letter via Kagaya-kun's crows telling him of the meeting that will be held weather or not he agrees. Reluctant, he went with the boy that he adopted and THEY DIDN'T KNOW THEY WERE BEING FOLLOWED.
That was the supposedly last battle was held ; in Muzan's mansion where the people that he helped resides. Now, remember that syringe that was shot at him? That was the icing on his father's cake. In the middle of the battle, he's little by little losing control of himself causing him to burden the others who were fighting alongside him. And his father was ecstatic! The bastard has the guts of the most feral gremlin ever, calling out to him to come to the dark side with him and whatnot.
Of course, that didn't happened, cause you know the boy that he adopted? He slit his own wrist in effort of getting him to go to their side instead of the demon king. And it worked, Muzan was now in full control of himself, except that he isn't. When he came by, when his senses came back, Kagaya Ubuyashiki-san's body was like a doughnut. Everyone's dead and his cousin is lying in his lap, whispering to him,
"Please, don't loose control... I'm sure we'll meet again... In the near future or a hundred decades even... Cousin, this is not your burden alone..."
And Kagaya-kun's last breath was drawn. Muzan went berserk and lost control over himself.
Now, if you read the manga, just imagine if after the explosion in Kagaya-kun's house is that his memories suddenly just came back to him...
And in Muzan's last moments, his memories and real objective just bursts inside of him, but looking at sun and his decaying figure, he knows that it's too late...