A wind is blowing along with the evening, in the forest that reeked of blood, that is the place where the battle take place.

The epic battle take place with hashiras :Rengoku , Tengen Uzui , Saremi , Hatako (Au) , along with Tanjiro , Zenitsu and Inosuke received a quest to kill High-demons that want to dominated the world.

And they are completed the mission kill these strong demons . However... they are unlucky that they meet a demon blocking the way

That demon had estimated power is highest level : Calamity . A legendary demon about 800 years old.  Demon slayer call them Demon Lords  : the highest rank of Demon , each one of them has an army of demons .

That demon is one of the demon lords .He killed a lot of people there and was so powerful that he could destroy a town (spoiler) ,and the battle took place near a town

Fortunately the population was evacuated from that town.

That battle was so strong that, it's destroyed many buildings, roads and even trees. The Hashira is trying to defeat that demon although they are tired :

Saremi : Shit ! Why that demon is too strong , Even my breathing is not effective form him , that thing is stupidly powerful!!

The unknown demon then transform one of his arm into 20 whip-like blade then shash every hashiras from all directions with incredible speed , some of them get scratches in that one slash but still block those slashes

Hatako (former hashira, Au) : (it's heavy ! His whip not a Joke at all , it not only fast but powerfull ! I would be lost my hand if i got hit one of his whip )

Tanjiro : We need a plan to kill this demon

The unknown demons : Ho ... Impressive ! They are strong as the rumors said by other demon lords! especially the kid with hanafuda earring, But not enough !

the blade on his arm and started to move faster and one of his turned into a chain full of spikes that aiming for the pillar of sound and slashing towards the sound pillar Tengen

Rengoku : watch his arm !

Uzui: Fuck !

Tanjiro: forth form : Burning bone, summer sun !

Tanjiro's slash move created three fierce circles of fire around and cut all the blades and chains around Uzui, Uzui then slashed all the blades near him but that unknown demon start to smile

suddenly, a sharp blade came and was so fast that it stabbed into tanjiro's stomach

Zenitsu and inosuke worried: Tanjiro !!!

Zenitsu and inosuke slashed that demon from two directions

Zenitsu : sixth form : Rumble and Flash...

Inosuke: You will pay !!! Eighth form : explosive rush !!!

Seeing two demon slayers rushing at the demon, he said with a very depressed expression: annoying pects...

that demon easily dodge the breaths of two demon slayers easily and gave them a kick that sent them flying

and tanjiro cough up blood, the rest of the hashiras are seriously injured from those slashes

The demon mocked: very well! Humans like you can able to stop me for a moment, but unfortunately, what you have fought with me so far has been nothing more than a boring match, I just using only 40% of my power but enough to kill all you and eat you in a minute.

Tanjiro : You bastard !!!

he then laugh and said while look at Tanjiro : you ! You are the most dangerous one that could kill demon lord in near future , but lucky for me , i finally can see your power ... Then i gave you a chance . Do you want to be a demon race like us ? come on, you will become stronger and do what you like ! eat human and drinks with us and made a big party! Do you want to become demon and join us ? Instead of killing you, you have a chance to choose of your own ! Join us of dead !

Tanjiro :then i choose...
Unknown demon: choose wisely :))
the moment the demon spoke and the unkhown demon head near to Tanjiro's to hear , khew that Tanjiro takes the opportunity , he used headbutt to smash the demon head .
Tanjiro headbutt is strong enough that made a small shockwave and the demon take two steps back and bleed a bit , Tanjiro shouted : i will ... Never to become a demon to a monster like you who killed thousands of people and devour them

After headbutt and felt insulted , the demon started angry : i... gave to you a great chance and you missed it ,not only that insult me . then Die !!!

Tanjiro smiled , in the moment the demon want to rip tanjiro head off , the hand of a man tthat grabbed that demon's wrist and stopped it ,
that demon looks at the hand from that person and wants to see who does this

He saw that the person who stopped his arm was a man with long hair that reached his back and that hair color was pure white and the eye ,and that man has red cat-like eye

(Image Muzan has white long hair)

The unkhown demon think :( A Demon ? , But why it want to stop me and why I don't see his presence ?!)

The man with white hair threaten : get your arm out of my little son !!

Feeling gripped so hurt and threatened, the demon replied: Who the Hell are You ?!

Then he used his foot to attack but before he could react , the while hair man grip the demon arm and tossed into the sky easily before the demon could use his leg. ( image being yeet in the air by Demon king :3)

Hearing familiar sounds and smells, Tanjiro looked up and saw the white-haired man approaching him.

The white hair with red eyes said: Son... are you okay

Tanjiro heard a familiar voice, Tanjiro replied: Father...

The Hashira after hearing tanjiro said that they look at tanjiro in the direction along with white haired person in . They cried because they were saved by someone they felt familiar with
Rengoku: Muzan Kibutsuji, the legend. , he here now to save us..

Muzan Then saw tanjiro injured and a small hole in his stomach he started feel anger inside him
And he said: Son ... who did these wounds, please tell me . I will kill the one who cause this ...

Tanjiro: it nothing ... Father , just a small would .

Muzan(in long-whited hair): i khow , you need to rest , also the paramedics will come. Leave the rest to me " He smiled at Tanjiro. "Let me deal with this demon and you better should keep the wound"
______________ Demon p.o.v ______________

After getting tossed in the sky by that shitty bastard , i looked around saw a few houses and wanted to find a place to land , but , where my arm and leg , i don't see , are they cutted and i don't still don't khow why are they are cutted like that and how fast of that demon to does it ? As a result ,i didn't holds myself well and the thrust is very strong, it pushed me and collided with houses and destroyed 5 houses nearby made me dizzy for a bit then I woke up.

i Regenerated all my limb quickly and in a anger voice :" who is fuck is that Oni ? How it is so fast and very strong ? why it want to protect the kid with hanafuda earring while it was a demon ?!

I , a demon lord like me that get toss around by that mere Demon ?? This is third time that i got humiliated easily . This bastard! There is no other choices

I realize this wants to change the battle : I must be kill and devour that bastard before it too late , no matter how much it will cost!!

I started jumping and went back to the direction where I was thrown

__________________3rd Pov_________________

Muzan : Hashiras ... All of you did well at your mission :)) I can see your perseverance and determination and it would be lucky that no one get killed , for kill some High-demons that want to dominated the world and thwart their plans ... I will gave all of you a rewards after that annoying mission

Uzui: It's such a honor to meet you . Like this task, it's not that difficult . It just unlucky enough to face a Demon lord.

Then the white haired man went to Tanjiro and said: You did well at your mission with the hashira, it seems you were seriously injured by that guy.

Suddenly , a wind sounded , unkown demon returned with a strange shape , he converted his arms into two giant and sharp blades : , he attacked the white haired man in a rage : You bastard!!!

That giant collision and slash was so powerful that it caused a huge explosion, covering an entire area including hashiras and tanjiro, inosuke, zenitsu.

Tanjiro : Dad !!

After that big smoke , the unknown demon think he completed killed him , But he realized that his arms were disintegrated while Muzan was unharmed and not even a scratch.

muzan : My son , could you leave the rest to me , Your daddy is here to protect you ! Do you want to leave of watching my fight and learn it ?

Unkhown demon : H-how ? ( how his body is too hard )

After a while, the demon's head fell into the white-haired man's hand, the unkhown demon didn't have time to see the muzan's movement, and the unkown demon's head was crushed by the Muzan.
However , the unkhown demon was a Demon lord , lost his head doesn't mean killed him.

Muzan :A Brat Demon like you want to interfere in our family , right? Seem like a brat like you need death wish !

After the demon completed regrow his Head and hear that voice , the demon growl and attack : I AM NOT A BRAT , You lowly DEMON !!

he continued to control his body using his blood and created a tornado along with a beam of energy.

unkhown demon : ( Blood demon art : Storm cutting edge! )

Muzan : hooo...

the big storm it cut a large piece of land along with buildings and trees..

Unkown demon : take that ! , What ?!where is him ? Where is other hashiras and that kid ? Are they disappeared?

In an instant, the hashira and the demon slaying team were transported to a far away place in the blink of an eye
even tanjiro , hashira who is strong and above upper moons they still cannot see muzan Move them to safe place in an instant

Muzan : now , you guys are safe I have created a barrier to protect everyone in that area, so.. son, do you want to see our fight..

Tanjiro said with a gentle smile :... Yes !

Muzan sighs : very well.

then Muzan Moves behind the demon so that the demon doesn't notice

Muzan: Took You so long... brat !

The demon startle and noticed muzan behind him so he backed away ...

He growl : I'm not a brat ! How dare a lowly demon like you call me like that !!

Muzan mocked : through the way you talk and your arrogance, I know that no matter how old you are, you mind still only a kid inside . You are holdback didn't you , You chicken ..

End of spolier of this Au.
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Tanjiro kamado (unkhown arc )

Tier : low-7C (small-town level) , higher with demon slayer mark ( Legend say he is training with Muzan , who is the first demon)

Attack potency and Durabilily :around low-7C , higher with demon slayer mark( small-town level) (
Can somewhat keep up with unkhown demon at 40% of his power

Speed : Sub-Relativistic ( can keep up with unkhown demon at 40% of it power)

Weakness: cannot lie convincingly , cannot always use dance of fire god infinity ( it cost more Stamina than any other breathing)

Unkhown demon ( one of demon lord)

Tier : low 7-C ( small-town level )at 40 % of it power , even higher

Attack potency and Durabilily: low 7-C (small-town level)at 40% ( could overpower all hashiras and tanjiro at one). Even higher

Speed : Sub-Relativistic at 40% (faster than tanjiro , zenitsu), even higher

Weakness: arrogant, Sun could turn him into ashes , wisteria flower.

Muzan Kibutsuji( former the first demon , True demon king , the legend himself , Former tanjiro father)

Tier :7-C (?) Town level

Edit 1 :Canon Muzan just high 8-A ( multi-block level) (speed :massively hypersonic +) and people wondering why this Au Muzan is stronger than canon Muzan is my secret

Attack potency and Durabilily : 7-C(?)( Town level)
Could crush unkhown demon head like nothing . Took a blow of unkhown demon without a scratch

Speed : Relativistic(?) Could ripped all unkhown demon limbs without being noticed, faster than all hashira , Tanjiro , even unkhown demon can't see , could Move all hashira and Tanjiro , zenitsu , inosuke in under a blink of eye

Weakness : (normally sun cannot kill him , even wisteria flower ) only that can really kill him is center of the sun of other stars (15 million degree is not the joke !!! )

Hashiras (8-A to high 8-A) (Post- training with ???)

Hakato Nabosuri {Oc} (former Frost hashira, unkhown arc)

(This picture is form fan art ,not mine )

Tier : 8-A , higher with mark ( city block level)

Attack potency and Durabilily: 8-A ( City block- level) , Higher with demon slayer mark

Speed : (massively hypersonic +) Compared with other Hashira

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Hey guys ~~~ this is my first KnB Au made by me

Write : April 18
Finished April 20
Remake 1 : april 22
Thank for Reading