This was just a fun thought that I got just now, it is short and I think will portray a lot. So enjoy.
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???????????: 1 month and 5 day after Tanjiro left:
This place was massive, in fact, this place is absolutely impossible. The entire place defied logic and physics, rooms were upside down and the furniture didn't seem to fall. Also, the number of rooms that were inside this 'building' just couldn't be as the rooms seemed to be going for eternity, one could say that the building seemed infinite.
And it was perfect for Muzans army. Every room insight, every crevice in over 5 km diameter was populated by demons. Though among these demons were individuals that wore armour, these are the demon generals and among them was the famed upper moons. Though for some reason these powerful demons that could wipe out entire armies were standing in a room far away from the army of demons.
In one room the demon generals and upper moons were arguing amongst themselves deciding what their action should be to contain this 'thing'. But soon all that bickering was halted when Nakime pulled on her strings and a door flew open. And upon the doors opening an individual entered, an elegant woman wearing a long black yukata, the generals not knowing who it was, drew or summoned their weapons, but the upper moons did not, a matter of fact they were on their knees. Once the generals saw this they looked at the women once more, while they had not seen her before she had pale skin and long sharp nails like a demon. But those eyes were undoubtedly Muzan Kibutsuji.
Muzan was pissed that these demons, his servants, had dared to raise their weapons against him. But he knew that he would need them for his war, though he did not need all of them. But before he could be killed some of them one of the upper moons spoke. "My lord, your 'pet' has gotten loose and is tearing apart the entire fortress. 'It' has already killed 5 demon generals and 100 demon soldiers." Nakime said.
"Killed?" Muzan said as he was confused as to why she said 'killed'. Demons can not kill each other as only a demon slayer blade and the sun could kill them.
"'It' is using a blood technique that creates fire from the blood that 'it' releases, the fire then causes damage that can not be healed." She said once more. Muzan knew that this would troublesome as this meant that capture was not possible as from when she was turned she would not follow his commands and the curse didn't have an effect on her. But he didn't want to kill her and without Kokushibo, Sans, his crazy brother and the 5 other generals in the fortress, Muzan did not want to risk a capture mission without all the strongest demons here.
So Muzan said, "Nakime, keep teleporting her throughout the fortress make sure she doesn't come anywhere near here."
Nakime nodded and so every 30 seconds she played a string, while she was doing that Muzan looked towards his/hers/it's/shapeshifters/attack helicopters upper moons and generals. "So any updates on the attack."
At first, no one moved forward to speak up but then one of the generals was pushed forward by the upper moon with pink-red hair, the general wanted to pick a fight with the upper moon but he soon heard Muzan say, "So?"
"Sans and his brother have-"
"I know what has happened to them, but I want to know about what the 5 other generals are doing," Muzan said as he interrupted the demon.
The general gulped and said, "Well, my lord, the generals are trying to get to the location but with a large number of demons with them, they need to spread out and slow down their movements to move towards the dem-"
But before he could finish his report upper moon 4 Nakime screamed in absolute agony, before any of them could question what happened a large explosion rattled the entire fortress. All the demons inside the room left to look at what happened and as soon as they looked outside into the inner core of the fortress they could see it wall burning. That is why Nakime was screaming or was as she was no unconscious. The fortress was like her body and since she is connected to the fortress every burning part was tearing at her mind. Though they were still paralysed that the fortress was burning and some of them thought it was beautiful.
But Muzan was mortified at the sheer destruction that lay before her/he. As he thought about the fact that this was the doing of only one demon. Muzan could also see that some demons were caught on fire and weren't regenerating and most were actually disappearing he could only think about the fact that he had unintentionally created another monster.
As soon as Muzan came to the realization Muzan could only think of escaping and so he turned around and said, "KILL THE MONSTER AND PUT OUT THE FIRE N-!!"
But before he could finish the sentence there in the flames came the monster that he created, there running at him was Nazuko Kamado. Gone was the small and fragile girl and in her place stood 'it' as it had the body of a fully grown woman, tattoos of vines and a horn representing her power.
Muzan was shell shocked to the point where he could not move and because all the demons were looking at him they could not see the monster coming at them. But instead of going to kill the upper moons and generals, Nezuko ran straight to Muzan.
Before any of them could react this monster in demon form had slammed into Muzan and through the door. But the thing was this door was the same door that leads to the open world and as the man looked at the horizon he could see the sun.
Muzan tried to push Nezuko off but she held on tighter but he soon stopped for a second when Nezuko said, "YoUr CoMiNg WiTh Me."
Nezuko said that in a whisper but Muzan heard it none the less and with the cynical smile Muzan looked towards his subordinate and screamed, "Help me!!!!!!!!!!!"
But it was too late the sun was out and the light was about to hit them. So they instead closed the door that leads to the outside world and with that sealed Muzan to his fate. The Demons inside could hear both the girl and Muzan scream though their screams were so loud that they couldn't tell Muzan and Nezuko's screams apart as they were both now shared the same fate.