July 16, 1913

After several delays, the day came for Nezuko to test out her Limited Sunlight Resistance. As everyone in the Butterfly House watched in anticipation, the three little Butterlfy Girls all shouted in encouragement, "You can do this, Nezuko-oneechan!"

Nezuko smiled through her bamboo muzzle before taking a step into the sunlight and closing her eyes. As she stepped fully into the sun, she felt no pain, no danger, and no alarm. Instead, she felt the warmth of the sunlight hit her body, a feeling she had not felt in a very long time. As she opened her eyes, she then practically ripped her muzzle off. "I can walk… Holy shit, I can walk…"

Not caring that her sister swore, Tanjiro wiped away tears of joy and told her, "Nezuko, we're one step closer to normal."

As he approached her, she turned around and told him, "Oniichan, it feels good to be in the light again. I haven't felt this in such a long time!"

"I'm glad to hear that," Tanjiro told her. "I'm so glad to hear that…"

"Yeah, I… I…" Suddenly, her smile turned into a frown as her expression changed from one of happiness to one of horror. "Oh my God… Oh my God, I remember it… I remember everything…"

"Nezuko, what's going on?" Tanjiro immediately became concerned as Zenitsu rushed out to join them.

"Oh my God," Nezuko said to herself as she knelt down on the ground. "The blood… The body parts, everywhere… I remember it all, oniichan. I remember that day perfectly…" She then began to take several deep breaths as a horrifying vision played in her head. "I see it. I see him going through the house. He got to me last. I tried to defend them, but…" She then began to cry. "I couldn't save them, oniichan. I couldn't save mom or any of our brothers or sisters. I'm sorry. I'm so sorry." As Tanjiro tried to reach for her, she shouted into the sky, "Damn it, it's all my fault! It should have been me, not the rest of them! It's not fair!"

As most of them went inside, horrified by what had happened, James also rushed to meet Nezuko. "Nezuko! Nezuko!"

"Nezuko," Tanjiro told her. "Please, don't blame yourself."

"No," she told him. "It is my fault… I remember now what happened…"

1911, The Night The Kamado Family Was Murdered

At exactly 7:36 PM, a visitor came to the front door of the small Kamado house. As he knocked on the door, Kie answered it. "Hello?"

Before her was a man who appeared to be in his twenties. He was well-dressed for the weather, and also well-dressed in general. "Sorry to bother you so late, but do you know the directions to the town near here?"

"As a matter of fact, I do," Kie replied happily. "You're only about fifteen minutes away by foot. Just keep going south. Are you sure you don't want to just make a stop here?"

"Mama," said Nezuko as she came to the door. "Who is it?"

"It's a traveler," she told her. "He's asking for directions."

The traveler looked to Nezuko and smiled. "I see you have a nice family here. It reminds me of my own kids back in my hometown. Is your husband around?"

"He's dead," Kie replied. "It's unfortunate what happened to him."

"I'm terribly sorry to hear that," the traveler replied with a bow. "I didn't mean to upset you."

"It's fine," Kie assured him. "So, do you want to sleep here for a night? The walk down can be a bit rocky and rough at night."

"I'll be alright," he assured her. "But thank you. I've made journeys like this by myself all the time. In fact, this is the first time I've had to ask for directions. I'm kind of embarrassed."

"It happens to the best of us," Nezuko assured the older man. "Don't worry."

"Well," he replied with the tip of his hat. "I'll be on my way. Thank you, madam." As he left, what the Kamado family did not know was that this was no traveler, and he was not heading into the town down below at all.

...

At 10:42 PM, the sound of a window breaking awoke the entire family from their slumber in the cold winter. As Kie and Nezuko were the first to stir awake, they both adjusted their eyes and saw a man enter the house through the broken window. Immediately, both jumped up and sprung into action. Nezuko gathered her younger siblings, telling them, "Get up! Someone's here!"

"Get the fuck out of my house," Kie shouted at the man as she grabbed a rifle from the wall and aimed it at him, firing a shot and hitting him in the shoulder. However, to her horror, the wound she made immediately regrew and disappeared. "What the fuck?! Don't tell me you're a-"

Suddenly, the visitor, who was in fact Muzan Kibutsuji, transformed his right arm into a tentacle and stabbed Kie in the stomach, going right through her. "I'm a demon, indeed. I'm Kibutsuji Muzan, but it's not like you'll live long enough to use my name." Her blood spilled all over the floor and covered the wall behind her. Within seconds, she was dead.

Several of the younger Kamado siblings cried and shouted for their mom as Nezuko yelled, "MAMA! NO!" She then yelled at Muzan, "You monster! How could you do that to her?!"

"Tell me your name," Muzan demanded of her as he picked up the rifle Kie had used and found some ammunition nearby before loading it up. It was an older musket, so it took him a little bit of time to load it, but once he did, he once again asked her, "Tell me your name now!"

"Nezuko," she replied. "Kamado Nezuko! Please, don't hurt anyone else! I'll do anything if you just let me brothers and sisters live!"

"Hmm," Muzan replied, thinking to himself. "A tempting deal. I do see potential in you."

"Potential for what?!"

"To become a demon," Muzan replied. "I could see it in your eyes, Kamado Nezuko. You could become a very powerful one, indeed."

"I don't give a fuck," Nezuko yelled, uncharacteristically swearing in front of her younger siblings. "If you have to turn me into a demon, do it! Just keep the rest of my family safe, please! Don't hurt them!"

"Fool," Muzan thought to himself. "As soon as she turns into a demon, her little brothers and sisters will become her first prey." He then nodded. "Very well. You shall become a demon, and I won't hurt your siblings." He then approached her as his nails on his right hand sharpened. "This may sting a bit, but don't panic."

As he scratched her neck, Nezuko suddenly felt a sharp pain throughout her body. She thought to herself as she stepped back from her siblings and felt a sinking feeling, "What have I done? My God, what have I done?!" She then screamed loudly as her siblings cowered in fear and her teeth sharpened. Her eyes began to dilate and change form as her skin also whitened significantly.

"Welcome to demonhood," Muzan congratulated her with slow clapping and a smile. "Kamado Nezu-"

Suddenly, he was cut off when Nezuko lunged at him, yelling angry gibberish and trying to claw at his neck. Instead of attacking and eating her siblings, Nezuko was instead trying to kill Muzan. One of her siblings, Takeo, yelled to her, "Get him, oneechan! Do it for mama!"

"Get the fuck off of me," Muzan yelled directly at her. "Why the fuck can't I kill you for defying me?!" Try as he might, he was unable to initiate demon self-destruction on her, and found that he had no control over her at all. After half a minute of struggling, Muzan finally came out on top and knocked Nezuko out with a blow to the head, but did not kill her. "I won't kill you. You still possess potential, but you are far too defiant." He then turned to the siblings. "As for you four.."

Hanako asked Muzan out of fear, "What are you gonna do to us?"

"DON'T LAY A HAND ON MY SISTER!" Takeo then grabbed an axe and tried to swing at Muzan, but Muzan punched him in the face and then grabbed him forcefully, twisting his head all the way around until his neck snapped, killing him, before tossing his body down on the floor. Nezuko began to wake up, and in response, Muzan stabbed her with a tentacle that formed out of his back, injecting her with a paralytic poison that did not harm her, but forced her to stay still and watch in horror as her siblings were murdered one by one.

As Haruka tried to run out with Rokuta, Muzan used a tentacle he spawned from his back to grab both of them and impale Haruka in her stomach and chest, killing her instantly. He then picked up Rokuta and used a knife to stab him in the chest and stomach several times before cutting his neck open, killing him.

Finally, it was Shigeru's turn to die. He grabbed the rifled musket and shot him just as he tried to climb out of the window Muzan had broken in from. Once he was shot, he screamed at him, "DON'T KILL ME, PLEASE! DON'T DO IT!"

Muzan ignored his pleas and ripped an arm off with one of his own arms before eating it in front of him. As he bled out from the bullet wound and the ripped off arm, Muzan told him as he swallowed some of his meal, "Your sister shouldn't have been defiant. If she complied, I wouldn't have done this to you all."

Nezuko then began to yell at Muzan, indicating the paralytic was beginning to wear off. As she yelled, she began to say specific words, which shocked Muzan. "YOU BASTARD! YOU BASTARD! I'M GONNA KILL YOU! YOU HEAR ME?! I'M GONNA KILL YOU!"

Shigeru had died of blood loss by this point. "You can talk already?! But you haven't consumed any human flesh!"

"DIE! DIE, YOU MONSTER!" As Nezuko tried to get up, Muzan once again knocked her out, this time delivering a harder blow so she would be asleep for longer. By now, the entire house was a bloody and gory mess of body parts, bodies, and scattered objects and weapons.

"Fucking Hell," Muzan said to himself as he took a deep breath. "That was way too fucking hard. I should not have had that much difficulty." He then kicked a wall out of frustration with himself. "Fuck this shit. This whole thing was a bust." As he let himself out of the house, he looked around to see if anyone noticed or heard what had happened. "Well, at least there's no neighbors. Nobody could hear them scream. Maybe when that Nezuko girl wakes up, she'll eat someone and join me then."

Present Day

"So that's what happened," Tanjiro said as Nezuko finishes recounting what had happened the night their family was killed in a patient room at the Butterfly House. "Nezuko… I had no idea…"

"I'm sorry," Nezuko once again apologized to her older brother. "I tried to save everyone, but I only made things worse."

"How many times do I have to tell you?" Tanjiro was saddened to hear her sister still blame herself. "It's nobody's fault but Muzan's. You were only trying to protect our brothers and sisters."

"Nezuko," Shinobu then said to her, having been moved deeply by the story. "I feel like I should be the one apologizing."

"Why so?"

"I want to apologize for doubting your loyalty to us," Shinobu admitted. "I had plans ready and made to kill you, because I did not trust you enough when you first came to us." She was clearly not proud to admit such a thing. "I can see now that I was wrong. The fact that you… You fought Muzan right after you became a demon, and that you did it to protect your siblings… That's someone I would fight alongside anytime. I would do anything to keep my own sisters safe, especially since I'm the only older sister they have left now that Kanae-oneesan is dead."

Nezuko nodded. "Shinobu-chan, it's fine. You didn't have to do that. I understand why you did it."

"Anyway," James then told Nezuko. "You need to learn to not blame yourself. Y'all are gonna drive yourself mad if you do. The only person ya need to be angry at is Muzan. He's the one who broke into your house and murdered your family, not you."

Zenitsu watched from the doorframe, feeling terrible for Nezuko. "I've never seen her so upset before," he thought to himself. "It pains me so much to see her cry. I could barely hold it in myself… But I have to be strong for her, not just as someone who loves her, but as a comrade!"

Inosuke also felt bad, watching from the same position Zenitsu was. "Monjiro's sister was really affected by this. I've never known what it was like to be raised by a family, but from what everyone says, losing your family is tough."

Tanjiro told Nezuko, "Nezuko, I'm proud to call you my little sister. Don't you ever forget that." He then gave her a kiss on the forehead. "You're a Kamado, and you are strong just like me. If you want a way to channel your anger into something productive, you can avenge our family with me."

"Become a slayer?" Nezuko was confused. "Me?"

"Correct," Shinobu confirmed. "The Oyakata-sama is willing to commission you as one and have you learn a breath. He is just awaiting your answer of yes or no."

Nezuko then stood up from the hospital bed. "Tell him he's got an answer, and it's yes. Muzan fucked with the wrong coal-sellers when he visited my house."

"I'll contact Urokodaki-san," Giyu then unexpectedly announced from the doorway, having dropped by. "You'll need a trainer, and he's the best we have."

"Tomioka-san," Shinobu asked him with a slightly annoyed tone. "Do you have to just stop by like that randomly without telling me?"

"Regardless," he followed up with, ignoring her. "Having Nezuko learn the Breath of Water would be our best bet. She can train with her brother, after all."

"I never thought of that," Tanjiro pondered. "That does sound like a good idea, Giyu-san."