Forever Forbidden/Muzan Kibutsuji x Y/n

The Swamp Demon

The breeze felt nice against your skin, the sun shining brightly. It seemed like the perfect day. You put a hand to your sword as you heard a noise, but it was only a small sparrow. You sighed and returned to walking toward your destination. As you walked further down the path, you heard small chatter. You carefully made your way toward the noise. It was a red haired boy, with a checkered haori, and a wooden box on his back. You then noticed the scar, the scar on the boy's forehead. You were overcome with the feeling of joy as you rushed toward the boy, pulling him into a tight embrace.
"Tanjiro!" You squealed, tightening your hold on him.
"Y/n? Is that you?" Tanjiro asked returning the embrace.
"Yes, yes it's me. How have you been?" You asked, pulling back from the embrace.
"I've been good. You look different, it's been what? Two years since we've last seen each other." Tanjiro solemnly stated, scratching the back of his neck.
"Yeah, since you were training with Urokodaki and I had to train with my parents I could never see you." You said remembering the last moments between you and Tanjiro before the departure.
"Hmm, I see you succeeded in learning Moon Breathing." Tanjiro said motioning toward your sword.
"Yeah I did. It was hard work but I had good teachers. What about you?" You asked.
"Urokadoki taught me many things, all that I use today. But Y/n, what are you doing here?" Tanjiro asked furrowing his eyebrows.
"I'm on a mission, young girls have been going missing with no trace." You sighed deeply at the thought.
"Oh, I'm going on the same mission. Seems it's fate we caught up to each other again." Tanjiro said with a small grin.
"It truly is. How's Nezuko? I've heard she's gotten stronger since Final Selection." You asked gazing at the wooden box.
"She's doing good, and indeed she has gotten stronger." Tanjiro stated smiling more brightly now.
"That's great news! Well, since we're going on the same mission let's work together. I want to catch up more with my childhood best friend." You asked with a smile plastered on your face.
"Sure, I would love that." Tanjiro replied.
You and Tanjiro spent the next hour walking and talking. You and Tanjiro talked about what's been going for the past few years. You've learned that a man called Sabito and a girl called Makomo helped Tanjiro in his training. But he ended up learning they were dead. You felt sad at that, you had wished to meet them. Once you reached your destination you saw a man swerving and stumbling his way through the street. You had caught some people off to the side of the road whispering about him.
"Look, Kazumi is so pitifully haggard because he was with Satoko when she disappeared." A woman whispered.
"Yes it's awful! When night comes another young girl will disappear." Another girl whispered.
You felt awful for the man. You couldn't imagine one of your close friends disappearing out of thin air, right before you. You quickly stepped in and turned to face the man.
"Kazumi! I would like to speak with you." You subtly yelled.
"Oh, uh, what do you need?" Kazumi mumbled.
"Please take me and my friend to where the young girl disappeared." You asked with a stern tone.
"Oh... Okay." Kazumi replied with a small frown.
Kazumi led you and Tanjiro through ally-ways and streets until you eventually arrived to where the young girl disappeared.
"This is where Satoko disappeared. You may not believe me but..." Kazumi quickly stated.
"We believe you." Tanjiro said with a small smile.
"Yes, we truly believe you Kazumi." You said with that same smile.
Tanjiro suddenly dropped to the ground and started sniffing the ground. You looked at him questionably until you realized he was trying to sniff out the demon. He always had a good sense of smell, but it seems it's improved. Tanjiro met your gaze, he nodded his head signaling to you that a demon had been here. You nodded back and sighed. You noticed Tanjiro stiffen and rise from the floor, quickly grabbing your wrist and tugging you along with him.
"The smell is getting stronger! It's a demon!" Tanjiro yelled as he let go of your wrist.
"Well let's go kill this bastard!" You yelled back with a grin visible on your face.
Tanjiro suddenly jumped up onto a roof. You smirked and jumped after him, closely following his lead. You and Tanjiro in sync dropped down from the roof and into a secluded street, no civilian in sight.
"Here It's here! Two different scents! A demon, and a human woman." Tanjiro said staring at the floor.
"In the floor?" You asked.
"The scent is uneven, it must be a demon that uses blood demon art." Tanjiro blankly replied.
Tanjiro suddenly stabbed the floor with his katana. The space around his sword turned into a black ink like substance. The waist of a woman wearing a kimono was soon revealed. You jumped into action and pulled the girl out from the black substance. You and Tanjiro suddenly jumped back as you saw another head emerge from the black substance. You quickly handed the girl to Tanjiro and pulled out your katana from it's sheath. Tanjiro's expression changed as he saw the color of your katana. Your katana was a shade of dark purple. You held your katana with a firm grip in front of you as you stared the demon down.
'Tanjiro was right, this demon uses supernatural abilities.' You thought.
"Where are the girls you kidnapped! Also, tell me about." Tanjiro asked
Tanjiro was interrupted by the demon screeching it's teeth together. It was a painful noise that made you cringe. Once the demon was done screeching it's teeth, it disappeared into the black substance.
"Kazumi! Take her and stay by me! If you stay inside me and Y/n's reach, we can protect you!" Tanjiro asked.
Tanjiro handed the girl to Kazumi and pulled out his katana, he stepped beside you and wielded his katana with both hands. You noticed his katana was a shade of black. You grinned and turned your attention back to the black substance. Like Tanjiro had a good sense of smell, you had a good way of sensing things. You could tell when a demon would attack even if it wasn't in front of you. You could sense they're attack and have enough time to deflect it. For example right now, you could sense the demon underneath the ground, ready to attack from below. Tanjiro seemed to notice as well, you noticed him get into attack stance so you quickly interrupted him.
"I've got this, protect Kazumi and the young girl! They'll need it!" You yelled.
You jumped into the air as you felt the demons rise from the ground beneath you.
"Moon Breathing, Fifth Form, Moon Spirit Calamitous Eddy." You breathed out.
Tanjiro watched in awe as you cut the demon's arms off. The only problem was they're were three demons now. You jumped back and eyed the demons cautiously.
'Fuck, I missed all the vital spots. I didn't realize they're were three demons!' You thought.
"Y/n, the demons all smell the same, and usually demons don't team up together. They must be one demon split into three." Tanjiro yelled.
"Got it!" You yelled back.
Suddenly one of the demons lunged at Kazumi and the young girl.
'Shit, I can't get there in time!' You thought.
Tanjiro was quick to act as he jumped into the air.
"Water Breathing, Second Form, Water Wheel." Tanjiro breathed out.
You watched as Tanjiro cut a opening into the demon's arm. You then realized, Tanjiro is a demon slayer as well. He doesn't need your protection, he can take care of himself. Tanjiro went to slash the demon again but it swerved away. You watched as the demon stared you and Tanjiro down.
"You bastard's! Don't interfere! You'll ruin the girl's freshness! She is already sixteen years old! She gets less and less tasty with each moment that passes!" The first demon screamed.
"I must calm down. Oh well, never mind. There's always tonight. I've eaten quite a few sixteen year old girls in this town. They were all meaty and quite tasty! Good enough for me." The second demon stated.
"Well, not good enough for me! I'm still hungry!" The first demon yelled.
"You monster, let Satoko go. You took her two nights ago!" Kazumi shakily yelled.
The demon that screeched it's teeth emerged from another spot of the black substance as the second one spoke.
"Satoko, who is that? If her hairpin is among this collection, then I've already eaten her." The second demon stated pulling out a piece of cloth with a bunch of hairpins attached to it.
You painfully watched as Kazumi's expression turned from scared to extreme sadness. Tears streamed down his face as he stared at the red bow in the collection.
Two demons lunged at you and Tanjiro. Tanjiro stabbed the wall which the demon was coming out of and you jumped into the air, rushing to the other side of the demon that was attempting to kill you.
"Moon Breathing, Ninth Form, Waning Moonswaths." You breathed out.
You cut the demon but again only it's arms. The demon was quick to regenerate and began to get ready to charge at you again.
'I missed the vital spots again!' You thought.
You quickly jumped away from the demon and quickly stole a glance at Tanjiro. He was struggling a little bit. You watched as he prepared for an attack but a demon lunged at him from behind. You were about to rush into action but Nezuko kicked the box open and kicked the demon's head so that it spun around.
"Why is a human carrying a demon?" The first demon stated disappearing into the ground.
They're were no demons in sight at the moment. All of them had disappeared into ground. You watched as Nezuko walked over to Kazumi and the young girl, she put her hands on the side of they're faces. Nezuko suddenly walked away from them and toward you. You grinned at the sight of her, she had gotten taller.
"You've grown." You stated proudly.
Nezuko blinked at you and then pulled you into a tight embrace. You grinned even brighter and returned the gesture. Tanjiro smiled at the pair of you, the long awaited reunion. Nezuko soon let go of you and steadily walked toward a puddle of the black substance. Her face changed, veins popped out and her pupils turned into demonic slits. Nezuko raised her leg and stomped on the ground, hard.
"Nezuko! Don't follow it! Come back!" Tanjiro yelled.
Nezuko turned her head and nodded. She started making her way over until another black puddle was shown and another demon emerged from it, lunging toward Nezuko. Although Nezuko wasn't worried at all, she simply jumped over the demon and ran to Tanjiro's side. Suddenly a black puddle emerged beneath Tanjiro.
"Y/n! Nezuko! I'm going under! Protect them!" Tanjiro yelled.
Within mere seconds Tanjiro was gone, and had been swallowed by the black substance.
"Nezuko you cover me when I'm down, alright?" You asked.
Nezuko nodded her head in response. A demon soon emerged from the ground. You took a deep breath and lunged toward the demon. You seemed to have caught the demon off guard as it was getting ready to attack Nezuko.
"Moon Breathing, Third Form, Loathsome Moon Chains." You breathed out.
You had cut off the demons arms and legs, you kept cursing under your breath knowing you were aiming for the neck. You jumped back as the demon regenerated one of it's legs. Nezuko stepped in as you backed over to Kazumi and the young girl. By the time Nezuko was attacking the demon it had regenerated it's missing body parts. Nezuko punched and kicked the demon until it's face was bloody. The demon had managed to slash three cuts on Nezuko's forehead. You were about to go and help her but Tanjiro beat you to it.
"Hands off my sister." Tanjiro sternly said making the demon retreat and fall to the ground, it's back against a wall.
You watched as Tanjiro stared at the demon and walked up to it, pointing his katana at it's neck.
"Tell me about Muzan Kibutsuji." Tanjiro demanded.

A/n: Thank you all for reading, I wanted to mention that this book is not all canon. I understand only a demon can use Moon Breathing but I decided they're would be another reason Muzan liked Y/n. Y/n was trained by her parents, because Y/n's family has a long history with Moon Breathing. So yes, Y/n is related to Kokushibo and Yorichi. That is all you need to know, good bye!