Mugen Train

Infinity castle is an alternate dimension consisting of endless wooden rooms, halls, and moving corridors. This castle has distorted gravity, allowing demons to stand upright, upside down, or perpendicular to the walls, or move in physically impossible directions.

The melodious sound of a biwa can be heard before the Lower Rank Kizuki appear in the Infinity Castle.

'Where is this?' Kamanue thought in confusion. He looked around the ever-moving castle and found a long-haired demon woman playing the biwa.

Each time the biwa was plucked, a member of the Lower Rank Kizuki would appear.

Based on this, Kamanue knew that this castle was the Blood Demon Art of that woman.

'Are all the Lower Rank Kizuki gathered? Why are the third and fifth ranks missing?' Kamanue thought.

The sound of the biwa continued, leaving the Lower Rank Kizuki confused. They kept observing their surroundings to see if the third and fifth rank members would appear.

As if sensing the boundless cold gazes, they lifted their heads and saw a red-eyed demon woman in a black kimono staring at them.

'Who is she?' Kamanue thought as he did not recognize the woman.

"Bow down and prostrate yourselves!" the woman commanded in a very cold tone.

Strangely, it was not a woman's voice that came from her mouth but the cold, emotionless voice of a man.

With that command, the bodies of the Lower Rank Kizuki immediately moved automatically and prostrated on the floor with their heads bowed.

Only then did they realize that the woman was Kibutsuji Muzan taking on another form.

"I apologize; your form and aura are so different..."

"Who allowed you to speak? Be silent!" As Mukago attempted to explain, Muzan interrupted and spoke coldly with a hint of anger on his expressionless face.

"Trash like you should not speak so freely. You are only permitted to answer my questions."

"Rui and Wakuraba have been killed. The only thing I want to ask is, why are the Lower Rank Kizuki so weak? Just because you have become a member of the Twelve Kizuki does not mean that everything ends there. In fact, everything is just beginning—a beginning to consume more humans. A beginning to become even stronger. A beginning to become someone useful to me." Kibutsuji Muzan said expressionlessly, leaving anyone unable to discern his emotions.

"It has been over a hundred years, and the members of the Upper Rank Kizuki have never changed. And how about the Lower Rank Kizuki? How many times have the members been replaced?" Muzan said in a chilling tone, with restrained anger.

'Even if you tell us...' Kamanue thought.

"Even if you tell us... what?" Muzan said coldly, echoing Kamanue's thoughts.

'He... can read thoughts?' Kamanue broke into a cold sweat.

"What's so urgent? Speak!" Muzan said, filled with rage.

"I beg your forgiveness, Kibutsuji-sama," Kamanue pleaded for mercy, but it was useless as Muzan killed all the Lower Rank Kizuki except for Enmu.

As the progenitor of demons, it was easy for Muzan to kill other demons even without a Nichirin blade. In front of the Demon Progenitor, their regenerative powers were useless and could not be wielded; they could only watch helplessly as death approached.

The reason Enmu survived was that unlike other demons, when he was about to be killed, there was no fear in his mind, only pure fanaticism towards Kibutsuji Muzan. This intrigued Muzan, granting him a chance, giving him plenty of his blood, and assigning him the task of killing Hashira and Tanjirou, who possessed the Yorichi Tsugikuni earrings.

.....

Sitting quietly in a room in a hidden house, Kagaya sat calmly and read the information presented by a Kakushi member seriously.

The report indicated that many people had gone missing while boarding the Mugen Train. Even the Demon Slayer Corps sent to investigate had disappeared without sending any news back. He knew that the demon capable of this was no ordinary demon, likely of Kizuki rank, which is why Kagaya had no hesitation in instructing a Kasugai crow to convey a mission to Rengoku Kyoujuro, Tanjirou, Zenitsu, and Inousuke.

Kagaya extended his hand, and the bird flew, soaring high into the night sky. Kagaya observed closely as the crow vanished from his sight.

Once the Kasugai crow disappeared, Kagaya began planning his next steps.

...

"Ca... ca... ca... It's morning; wake up." In the morning, Tanjirou's Kasugai crow appeared at the window of Tanjirou and the others' room, waking them.

"The victims of the Mugen Train are increasing. Now 40 people have gone missing. You are requested to go to the scene with Rengoku Kyoujuro. Depart to the west immediately!" The crow conveyed the mission as Tanjirou and the others were waking up.

Tanjirou listened intently.

On the other side, Haru, who was sleeping in the room next to Tanjirou, also heard the mission and immediately woke up. Wearing her Demon Slayer uniform and white Haori, Haru approached the Kakushi member assigned at Butterfly Mansion and asked him to take her to Kagaya.

Haru insisted on quickly stating that there was something important related to the Upper Rank Kizuki, so she needed to meet Kagaya right away.

Faced with the request of a Hashira and the seriousness of the matter, the Kakushi quickly summoned a Kasugai crow to inform Kagaya of Haru's arrival before taking her to where Kagaya was.

After going through many winding paths, Haru finally arrived at a simple mansion in a valley far from the city.

"Haru, I hear you have something important regarding the Upper Rank you want to convey," Kagaya said as she sat upright across from Haru, with his wife, Amane, behind him.

"Kagaya-sama, I hope you believe what I will say next," Haru said seriously.

"Then speak," Kagaya said seriously as he sensed Haru's determination.

"Kagaya-sama, I wish to participate in the Mugen Train mission, and I also want to bring the Stone Hashira along. On the Mugen Train, besides the Lower Rank Kizuki, there is also Upper Rank 3, Akaza, who will appear. Unlike the Lower Rank, the Upper Rank can only be fought if three Hashira work together," Haru said seriously.

"Oh? Are you sure? Where did you get that information?" Kagaya asked seriously.

"At least 80% of Akaza's appearance is accurate. Recently, I awakened a special ability to see fragments of the future, but this ability will only appear randomly," Haru did not fully lie, as knowledge of the plot and the ability to see parts of the future were true.

Kagaya was momentarily surprised after hearing Haru's answer but had no doubt that such an ability indeed existed, as he had a similar ability. This world was simple; perhaps there was something deeper hidden beneath. The mysteries of this world were no less than those of other supernatural worlds, evidenced by the existence of souls like Makomo and Sabito who always resided in Mount Sagiri alongside Urokodaki, even accompanying and teaching Tanjirou the water breathing techniques. Additionally, the presence of the priest clan who always married into the Ubuyashiki family to prevent the early death of Ubuyashiki family members was also very mysterious.

"I see, is there anything else you saw in the future?" Kagaya asked with a dignified expression.

"I saw the death of Rengoku Kyoujuro in this battle, therefore I hope to participate in this battle with the Stone Hashira. With the addition of the Stone Hashira, we have a chance to kill the Upper Rank 3"

"Okay, I will immediately convey the news to Gyomei and ask Rengoku to wait for your arrival." Kagaya took a piece of paper before writing something on it and asked the crow Kasugai to send it.

"Thank you for the information, Haru" Kagaya said gratefully to Haru.

"No, as a Hashira, I also have a responsibility to fight demons and there's no way I can just keep quiet when I know my friends will die," Haru answered firmly.

"Then, I'll take your leave first, Kagaya-sama" Haru bowed before leaving the room.

...

In the midst of the bustling station filled with the sounds of train whistles and the footsteps of passengers, Inousuke stood tall, wearing a boar mask that covered his head. He gazed at the steam train that had just come to a stop, his eyes burning with the desire to fight, as if it were a giant beast dominating the land.

"What is this?! Is it the ruler of this land?!" shouted Inousuke, his voice echoing through the crowd. All eyes turned to him, some in disbelief and others in amusement.

Zenitsu, standing beside him, could only shake his head while hiding his face behind his hands. "Oi, Inousuke! I told you, this is a train!" Zenitsu tried to explain in a panicked tone, feeling embarrassed by his friend's outrageous behavior.

Ignoring Zenitsu's cries, Inousuke grabbed his sword with fiery enthusiasm and stepped forward.

"If I defeat you, I will become the ruler of this land!" he declared, brimming with confidence, before banging his head against the side of the steel train. The sound of the impact echoed, startling several people nearby.

Suddenly, a station officer in a formal uniform approached them with firm strides.

"Hey, what are you doing over there? Put down that sword!" he shouted. He glared at Inousuke, who seemed intent on slashing the train, clearly displeased.

Zenitsu, noticing the situation escalating, quickly pulled Inousuke away.

"Let's go! We can't deal with these people!" he said while running off, followed by Tanjirou.

In modern Japan, carrying a sword is strictly prohibited. Even though the Demon Slayer Corps fights against demons to protect humanity, their status as an illegal organization forces them to conceal their weapons whenever they are in public spaces.

After escaping the station officer, they finally bought tickets and boarded the train.

"Delicious!!!"

Upon entering the train, they searched for Rengoku Kyoujuro when they heard an enthusiastic shout. Turning, they saw Rengoku Kyoujuro, with hair yellow-tipped red like fire, devouring food while continuously exclaiming "Delicious."

Besides Kyoujuro, they also spotted Haru, who was eating silently, and Gyomei, who was sitting quietly.

"Haru, why are you here?" Tanjirou asked in surprise, as he thought only he, Zenitsu, Inousuke, and Kyoujuro were assigned to this mission.

"There's been a change in the mission, so I and Himejima-san will assist," Haru replied, setting down her lunchbox.

"Change?" Tanjirou asked, puzzled.

"You'll find out later," Noah did not elaborate, knowing they were currently inside Enmu's body, which had merged with the train. He was unsure whether Enmu could monitor every corner of the train, but for safety, it was better not to discuss Akaza.

"I have something to ask Rengoku-san. It's about my father," Tanjirou said to Kyoujuro.

"What about your father?" Rengoku Kyoujuro asked loudly as usual.

"When he was alive, he was often ill. However, he could perform a dance in the freezing snow," Tanjirou shared a bit about his father.

"I see, that's good to hear," Kyoujuro responded with his usual fiery enthusiasm.

"About the Dance of the Fire God Kagura, without realizing it, while fighting on Mount Natagumo, I used a dance I saw when I was young. If Rengoku-san knows anything, please tell me," Tanjirou said.

"Yes. Unfortunately, I don't know!!"

At first, Tanjirou thought Rengoku might know when he heard his answer, but he was taken aback by the second part of Rengoku Kyoujuro's optimistic response.

"Tanjirou, if you want to know, I will tell you," Haru interjected, having overheard their conversation.

"Haru, do you know?" Tanjirou asked with sparkling eyes.

"Of course. The Dance of the Fire God Kagura has another name; it is known as the original breathing technique..." Haru began.

"Haru, you mean..." Rengoku looked at Haru with an astonished expression.

"That's right, the Dance of the Fire God Kagura is a long-lost Sun Breathing technique," Haru confirmed.

"Really? If we could master this technique, the strength of the Demon Slayer Corps would become even greater," Kyoujuro said excitedly.

"Unfortunately, that's not possible, Kyoujuro. Not everyone can learn it; otherwise, how did the five breathing techniques of Water, Stone, Wind, Lightning, and Fire emerge if everyone could learn it? The reason those five techniques appeared is that only Yorichi Tsugikuni could train them, which is why the demon slayers hundreds of years ago created the five breathing techniques based on the Sun Breathing technique under Yorichi Tsugikuni's guidance," Haru explained patiently how the origins of the current breathing techniques came to be.

"I see, what a shame," Kyoujuro sighed.

As they spoke, the train conductor came to check tickets. Haru knew that once the tickets were checked, they would soon fall into the dream created by Enmu, as Enmu's Blood Demon Art was attached to the tickets, and once the tickets were punched, the Blood Demon Art would activate.

Even though she knew they would be trapped in a dream, Haru still handed over her ticket for inspection to avoid raising Enmu's suspicions. After all, Haru's main target was not Enmu, but Akaza.

As soon as the ticket was punched, Haru unknowingly fell into the dream. The scenery around her changed; it was no longer the Mugen Train, but a traditional Japanese house that felt very familiar to her.

Haru never expected that it was not her first life she longed for, but Sora, her younger sister in her second life.

"Haru," Sora's pouting voice woke Haru from her daze.