Kimetsu no Yaiba/Demon Slayer Isekai. Female lead.

Defeat

Muzan had gathered his remaining upper moons and demons into the infinity castle. He planned to make his move. He had discovered the location of the Ubuyashiki Manor and planned to eradicate the demon slayers. Then he would have all the time he needed to figure out the secrets of Haku's blood without worry of any interference. All Haku could do was hope that the weapons made with her blood, the cures, and the power it would give the hashira would be enough. Muzan was dressed to impress she supposed, he donned a white suit and fedora. He took Haku to a room within the fortress along with Kokushibo, who he placed in charge of her. She sat alone in a small room with her hands and feet bound. Kokushibo was close by, awaiting any slayers who dared cross his path. Haku tried to focus. She stood no chance against Kokushibo alone, but if the others came, she would do whatever it took. For the time being, she would work at loosening herself from the ropes. Her wrists started to sting as the rope dug into her skin. She felt the wetness of blood. Maybe it would help loosen the ropes.

Akaza dove into the familiar yet ever changing infinity castle. He rushed ahead of the slayers that had entered at the same time as him. Kagaya had managed to stall for enough time when Muzan arrived at the manor for the hashira and other slayers alike to arrive. Tamayo had then made her move, forcing several vials of the cure and poison into Muzan. It immediately took effect, and started tearing the demon apart from the inside. How the Kamado girl survived her change back to human, Akaza did not know. Muzan had then retreated into the infinity castle. He dodged around the moving parts of the castle. Nakime must have had it out for him. He then caught her scent, the scent of Haku's blood. Was she injured? He made his way in the direction of her scent. If the rooms changed, it did not matter. He could locate her now.

Haku could hear the sounds of many slayers in the halls, mostly dying screams. She tried to hurry with the ropes, but they were tied tight. She tried to swing her sheathed katana around in a way she could unsheath it to cut the rope, but to no avail. Suddenly, the door to her room caved in and Akaza appeared, rushing straight for her and slicing the ropes. Haku jumped up and hugged him. "Akaza! Thanks." He gingerly returned her embrace. The moment was quickly broken as Kokoshibo entered the room. He looked at Akaza with a cool expression, but seemed mildly irked at the state of Haku's wrists. "It would seem we are destined to battle again. I did enjoy our fight before, but this time will be your death." Akaza cocked his head to the side. "I'm much stronger than before, you know." Haku agreed he was stronger than before, probably stronger than Douma now, but she knew he was still not at Kokoshibo's level. The infinity castle moved and Haku heard the sound of someone being thrown into the room. She looked at the slayer that had arrived. "Muichiro!" She recognised him immediately. Muichiro stood up and smiled at Haku. "Haku! Saves me from searching so much. Seems the demon guy found you first. Who's this other guy?" He looked at Kokushibo. Muichiro's whole body stiffened and he looked unnerved. The aura Kokoshibo gave off was spine chilling. He quickly gathered his composure. Kokushibo eyed him intensely. "Ah, I see now. You are my offspring. What is your name?" He asked. Muichiro readied his blade. "Tokito Muichiro. What do you mean offspring?" Kokoshibo blinked. "So the Tsugikuni name is gone. I mean that you are children of my children. Perhaps a little farther as it has been over 400 years." He explained. Muichiro looked peeved. He lunged at Kokoshibo, using his mist breathing. Haku moved quickly and snatched him back, just before Kokoshibo's blade met his arm. Kokoshibo looked at Haku. "Do not interfere. You are to be kept alive for the master, but I'm capable of incapacitating you though it will be rather unpleasant for you." he warned. Haku snarled at him. "I'm perfectly ready to dive into oblivion and die before allowing Muzan to use me." Kokoshibo looked very displeased. Haku slipped her hand into Muichiro's pocket and pulled out the vial of her blood he had been given. "Drink it. You're gonna have to." Muichiro was not a fool and he downed the blood quickly. A shot rang out and Kokoshibo easily dodged the attack. Genya came from around the corner and Kokushhibo quickly cut him in half. Genya lay bleeding but alive since he had eaten demons. Haku hurried to his side, and managed to block Kokoshibo's next attack that was aimed to kill. Sanemi then came in and sent the demon backwards. Sanemi had a wild smile and his mark was already active. He looked at Haku and his brother. "That blood is damn good stuff. Haku take care of him." Haku nodded. Muichiro and Sanemi both started their attack. Akaza jumped into the air and joined with well aimed air fist attacks. Haku had a piece of Kokoshibo's hair and gave it to Genya. "I didn't play good little captive for nothing Genya. Upper moon one's hair. A serious piece of power plus my blood should equal awesome. I hope."

Genya was able to awaken a blood demon art with the combination. He was able to immobilize Kokoshibo with it. All of them danced around fighting back his moon breathing slashes until finally Sanemi got the cut, beheading Kokoshibo. He then morphed into a monster. Haku stood directly in front of him and stared into his eyes. "A real warrior dies with pride when defeated!" She stepped up to him and pulled the flute from his pocket. "Your brother kept this because he loved you. He cried for you the day he died because he loved you." Kokoshibo froze, and by his own choice, faded to dust. Everyone looked at each other and smiled. They were bloody and bruised but all alive. The castle then started to crumble. Everyone rushed to make their exit from the infinity castle. It was still night outside. Muzan was there, in a grotesque form. Tanjiro was using the hinokami kagura on him as much as he could handle. Haku was breathing heavily, her own blood covering her sword. The sun started to peek over the horizon. They could finish him. She looked at Akaza to tell him to go hide from the sun. The demon grabbed her hand. "I remembered my past. Listen, I have lived a long time now and done so many bad things. I just want to see the sun one last time." Haku looked at him and started to argue. He placed his finger on her lip. "Kyojuro will make you happy and protect you, so I know you will be safe. If I am ever reborn, I hope to see you both again." He let go of Haku's hand and rushed to help Tanjiro and the other hashira with Muzan. Everyone was pouring their all into the assault. Haku could see Kyojuro battling as well and he smiled when he saw Haku. Haku started to cry. If they could just do this, just hold out, the future would be different. She activated her Rokkon Shojo and joined the fray, the mark on her neck blazing to life. That was when her sword turned red. She moved into Muzans body "Inyo breathing: twelfth form: Inyodo!" She dashed through and cut all of Muzan's hearts and brains then moved to behead him. He fell to the ground, destroyed and beheaded. His flesh mutated into a large fetus looking creature and sucked Haku into it. She heard Akaza and Kyojuro scream her name. It felt so strange. Soon she was face to face with Muzan;s head inside the lump of flesh. "I will not allow you all to win. I'm burning in the sun and iI will die soon, but I will leave them with the worst monster of all. If I turn you into a demon you will surely kill them all." Haku struggled and spat curses at Muzan, but he shoved a finger into her neck, pumping all of his blood into her. She waited for the pain of changing but it never came. Muzan looked beyond shocked. Her blood was not allowing Muzan's to take over. Haku gripped her blade tightly. "I will not allow you to survive. I will not become a demon. You end here Muzan!" Her blade ignited in a blue flame. Her blood had ignited from the heat of her blade. She managed to worm her way from the flesh and lunged at Muzan's head. "Secret Breathing Technique: Blue Spider Lily!" her flame engulfed all of Muzan, burning him into dust. She stood in the sunlight now. Akaza was standing a few feet away. He smiled one last time as he turned to dust as well. Tears ran down Haku's cheeks.

She collapsed, but before she hit the ground she felt arms catch her. She looked into the face of Rengoku Kyojuro and smiled. She was so glad he was safe. Kyojuro leaned in closer to her. "Haku, I am so glad you're safe." Haku reached up and caressed his face. "Is it really over?" she asked him. He smiled. "Yes." She closed her eyes and fell unconscious.