WRITTEN BY RIYA DUBEY
Kibutsuji Muzan is the demon king and the first of his kind. He is the strongest demon and is nearly unkillable.
The only person who was able to give lasting damage to Muzan was Yoriichi Tsugikuni and ever since then, no one has been able to harm him.
The upper moons of the 12 Kizuki are the strongest weapons that Muzan has in his arsenal. Each demon has his own blood demon art and is quite strong.
These demons are the ones that Muzan somewhat trusts and they are quite loyal to him. Therefore it is nearly impossible that they would turn on him.
However, in a hypothetical battle between Muzan and the upper moons, Muzan would most definitely win.
Kibutsuji Muzan is the creator of these demons. Only Kokushibo, Douma, and Akaza together would be a bit of a challenge for him, but he would still be able to defeat them.
In addition to that, Muzan has multiple hearts and brains, unlike any other demon. This makes it very difficult for anyone to kill him. The only surefire way is through sunlight which is something that the upper moons cannot utilize.
Therefore, even though it would be an amazing fight, Muzan would win against all of the upper moons.