What if tanjiro got the carnage symbiote?

The Demon kings full power

Kokushibo raises his head and widens his eyes in shock.
Kokushibo:Wh-what?
Carnage:You've taken countless lives,like so many others,demon and human alike.But unlike the rest,you hold a kindling,a kindling of chance,I know regret when I see it.In our final moments,our truth emerges from the since of these lies that place the world and the honest. Your truth,it's out and I can see it in your face. You have regret and sadness,you wish things would have gone differently,am I wrong?
Kokushibo ponders what he just heard.He...he DOES regret everything he's done,every choice he's made,and every bit of envy now becomes regret within kokushibo's heart,tears begin to fall form all of the demons eyes.
Kokushibo:I-I-I...I will serve you,master.
Carnage:Good,and now.
Carnage pulls out the device,he injects himself with it one last time as he then gives it to kokushibo.
Carnage:With this,you will be free from muzan's control.
Kokushibo takes the device and injects himself with it,as he does,he feels a sort of severance between him and Muzan and feels a sort of... liberation writhing his body.Kokushibo stands and forms another blade.Carnage puts his weapons away,the two look around and realize that they are out of the realm of infinity.
Kokushibo:Now what?
Carnage:Now,we find and kill Muzan.
Suddenly,a familiar voice comes from the duo's left,behind numerous trees.
???:No need for that.
Carnage & Kokushibo:!
???:I'm already HERE!
The mysterious adversary suddenly charges full force at the duo like some sort of missile, destroying the nearby trees and boulders. Carnage and kokushibo act quickly and leap out of the way,but are still flabbergasted by the force.
Carnage:M-Muzan?!But how?!He wasn't capable of this before.
???:Oh...
The dust clears to reveal-

Muzan:I'm capable of much more than that!
Carnage:Muzan?!What is that!?
Kokushibo:That is lord Muzan's full power.For him to be pushed to these lengths,your comrades must have really pushed him back.
Carnage:My comrades...what have you done Muzan?!
Muzan:Simple,I'm letting my pet take care of them now.They should be dead in no time.
Carnage:You bastard!
Kokushibo:Shall we?
Carnage:Yes.
Muzan:Kokushibo,you dare to betray me?I MADE you,I gave you life!
Kokushibo:And now I'll take yours,life just isn't fair is it Muzan?
Muzan runs full speed at the duo as they do the same.The three clash,and they clash intensely.
Kokushibo and carnage kick Muzan away,but the devil runs back at high speeds and creates arm whips of bone and meat.Muzan swings and clashes with carnage and kokushibo,but cannot overpower them.Muzan gets frustrated as he yells and makes a shockwave that sends carnage and kokushibo flying.
Muzan(thinking):Dammit!I was already a match for him before,so why are we completely even in strength now!?How is he constantly getting stronger AND faster?!
Muzan's thoughts are interrupted as kokushibo and carnage lunge at Muzan with their swords.
Kokushibo:Moon breathing!
Carnage:Darkness breathing!
Kokushibo:Third form,Loathsome Moon Chains!
Kokushibo slashed at muzan's body twice,as his arms swing crescent blades shoot from his arms and shred muzan's symbiote body.
Carnage:Second form,lasher!
Carnage wraps tendrils around himself and then swings,slicing Muzan's entire body with tendrils and at his neck with his twin swords. Muzan regenerates and kicks carnage in the chin and then gut punches kokushibo.Muzan takes a deep breath and creates another shockwave that sends the two flying,buying himself time.The two fly away before eventually recovering,but as they do,they look around and see the demon slayer corps. Everyone was injured,even Grendel,who tiredly laid on the ground with burn wounds.Rui runs up to the duo.
Rui:Master!
Carnage:Rui!What's the status on everyone else?
Rui:So far everyone IS alive,but Muzan ran away and made his dragon fight us when the infinity castle just fell around us.
Inosuke:TANJIRO!
Carnage looks to his left to see inosuke,Genya in his human form,Zenitsu,and Mitsuri.
Carnage:Everyone,you're alive,I'm so glad!
Carnage reunites with his friends.But the reunion is cut short as Muzan yells from the skies and lands in front of the entire group.
Muzan:Oh look,the gang's all here.Think if it this way,you at least can all die together.
Zenitsu:Be quiet,being talkative won't save you.
Inosuke:Hehehehehe,I like Benitsu when he's mad!
Genya:If this really is the end,then I need you all to make sure I can get to him,if I can eat some of his flesh we should be able to win easily.
Carnage:Sounds good,everyone!Get Genya close!
Group:RIGHT!
The group leaps at Muzan,who creates and sends out an array of whips.The group is thankfully able to dodge the saws. Mitsuri lifts Genya off of his feet as carnage lunges at Muzan.Carnage covers his entire arm in electricity and punches Muzan's shoulder, actually destroying some of the symbiote and exposing his shoulder.
Muzan:NO!
Mitsuri:Now genya!
Mitsuri throws Genya using all of his strength. Genya opens his mouth wide and latche so to Muzan.
Muzan:Hey!Get off!
Genya:Suck my dick!
Genya chomps down on Muzan's shoulder. Genya leaps back as he then undergoes his transformation.
Genya:Alright then!Let's end this!
Muzan:Grrrgrgr!Damn you all!You pests!
Muzan rushes at the group,but that was a mistake.Mitsuri and Kokushibo swarm Muzan with slashes that slice his body all over.Muzan tries to throw a slash,but inosuke swings his swords and cuts the whip off of Muzan's body. Zenitsu rushes at Muzan slashes his face and legs up,bringing him to his knees.Genya and Rui rush at Muzan and both throw their strongest uppercuts and send Muzan flying into the air.Genya unloads multiple shots into the demon's flying body.Carnage leaps into the air and brings his arms together,forming a large spike mace.The mace sparks with electricity as carnage swings it down upon Muzan like a hammer,sending him flying into the ground and cratering it.As the group comes back together,Muzan looks to his right to see Grendel weakly crawling to him.
Grendel:Guuuhh...
The dragon extends its claw as it unravels.
Muzan:You...hehehehe...HAHAHAHAHAHAHAAHHAHAAHA!
Muzan puts his arm out gleefully as Grendel's entire body suddenly completely melts and leaps onto Muzan,the dark substance swirls and expands Carnage realizes what they're doing.
Carnage:NO!
Genya:What the hell are they doing?!
The mass shrinks down and wraps into a large, bulky,and black demonic monster.It's eyes glow red as it forms a mouth as well as horns. Red lines and marks spread around his body. The being steps out and smiles.

Muzan:Heheheheh...HAHAHAHAHAHAHA!