3rd POV. At a hidden location.

So, after Kagaya, along with Amane and their two eldest daughters, Hinaki and Nichika died......

Who took over the Ubuyashiki Family?

It was none other than Ubuyashiki Kiriya, only son of Kagaya. As of now, he is being supported by his remaining sisters, Kuina and Kanata.

They are currently providing intel support and giving commands to the demon slayers who were brought into the castle by spreading the paper tags that Yushiro distributed.

It was through Sora's suggestion to use the papers as a better way to communicate and even coordinate with others.

Sora even mentioned that it works like the telephone, but without directly speaking to each other.

Since Sora almost spent most of his time in the city due to his past work, he was able to encounter a lot of inventions from the West.

With his information and even creating new tactics in battle due to reading a lot of books of warfare from China, and the West, specifically from the Napoleonic Wars and from the Roman Empire.......

Sora employed some of the Western warfare strategies, into their own. So during the Hashira Training, the demon slayers were taught how to coordinate with the movement of others, diversion, ambush, and employ what the slayers learned throughout the other previous Hashira trainings.

This is the training that Sora provided, Tactical Warfare. However, unlike others, he didn't taught it directly.

But he taught it by sharing these strategies to the other Hashira......and especially, to Kiriya, as Sora knew that he would be Kagaya's successor.

And now......

Kiriya, who is now taking command.......

Kiriya: Sora finally arrived to assist Kanae and Shinobu.

Kanata: But Upper Three is after Sora. Tanjiro, Giyuu, and Kyojuro are currently out and recovering.

Kiriya: Sora is buying time to let them recover.

Kuina: Kanao and Inosuke has arrived to assist the Kocho Sisters in battle.

Kiriya: Good. Sora believes that they will win.

Kanata: What about Sora himself? Can he win against Akaza all by himself? He got to conserve his strength in fighting Muzan.

Kanata: He is one of our best chances to defeat him.

Kiriya: Like I said, Sora is buying time. He knows that he can't afford to exhaust himself when he begins to fight Muzan.

Kiriya: Sora probably has a hidden trump card against Muzan.

Kiriya: I know it.

Kiriya: Once he got the opening......

Kiriya: Tanjiro, Giyuu, Kyojuro.

Kiriya: They got to hurry and assist Sora.

Inside the dimensional fortress.

Sora is currently holding off against Akaza, as he wants to minimize using his strength because he knows that he will have to go all out in fighting Muzan.

Akaza: Destructive Death!

Akaza: Disorder!

Akaza performs a barrage of punches straight at Sora.......

Sora: Hi no Kokyu, Shakkotsu En'yo.

Sora intercepts all the attacks by performing a large circular slash that is meant to defend.......

Akaza: Crown Splitter!

Akaza went for a reverse, rising axe kick. With the Mangekyo Sharingan's improved accuracy, Sora defends from it.

Sora: Hi no kokyu, Gen'nichi Ko.

Akaza: He disappeared!? Again!? But it's different—

Sora comes from behind and took a chance to attack Akaza.

Sora: Hono no Kokyu, Ichi no kata: Shiranui!

Sora performs a strong horizontal strike that was strong and fast enough that Akaza's leg was severed.

Akaza: What is this!? He's even stronger than before!? Even stronger than when Kyojuro and Giyuu got marked!?

Akaza: Not to mention, his fighting spirit! It's so faint! No, not faint it feels all over the place!?

Akaza: Is it because his eyes can track my fists and my foot!? My movements!? Not to mention, he can even copy the swordsmanship of Tanjiro, Giyuu, and Kyojuro.

Akaza: As if I'm just fighting them all at once but this time, a single person, with much superior skill and precision!

Akaza flips and lands on the floor.

Akaza: Hahahaha! You really should have become a demon like Madara.

Akaza: Unfortunately, he was killed.......

Sora: What if I tell you that I was the one who killed him?

Akaza: Oh.....so the descendant surpassed the ancestor?

Akaza: I knew it. You really are in the realm of the highest!

Akaza: Descended from the Goddess of the Sun.

Akaza: Possessing the eyes that can see the truth clearly......!

Akaza: You may not know this but I also fought many Uchiha before.

Akaza: Not only just Madara.

Akaza: But unlike the other Uchiha and Madara.......

Akaza: You, Sora, have refined your swordsmanship. Even refining it even in the midst of battle.

Akaza: Ah! That talent! That is the reason why I wanted YOU to become a demon!

Sora: *sigh*

Sora: I already told you. The answer is no.

Akaza: *sigh*

Akaza: You really are a stubborn one. I have no choice but to relish this fight to the fullest!

Akaza: Destructive Death!

Akaza: Eight-Layered Demon Core!

Similar to Akaza's Disorder......however, eight powerful punches that unleashes much powerful shockwaves.

Sora: Hi no Kokyu, Hekira no Ten!

By repelling the attack, Akaza was in the air but he didn't stop.

Akaza: Air Type!

This time, sending powerful shockwaves from the air.

Sora: Hono no Kokyu, Shi no kata: Sei En no Uneri!

Similar to the Breath of the Sun's Shakkotsu En'yo, the fourth form of the Flame Breathing, swings their blade in a circular manner but in different angles as it intends to defend from multiple attacks.

Akaza then goes for a powerful strike that Sora evaded.

Akaza: Saishiki: Manyosen Yanagi!

Akaza's attack cracked the ground and even Sora felt that if he didn't dodged that, he's done for.

Sora: So he has even more powerful techniques! If I haven't separated him from Doma he would be the problem!

Sora: Not to mention, he's always aiming for my vitals before he used his techniques. Just like before.

Sora: I was only able to evade and intercept them with the Mangekyo Sharingan's ability to grant me fluidity in battle and even anticipate where he would attack.

Sora: Say, he can also notice where I'm attacking despite me getting him off guard more than once.

Sora: However, that's the thing. According to what Kyojuro told me when he initially fought Akaza.....

Sora: Kyojuro felt that Akaza can read movements or even feel them.

Sora: It is similar to Inosuke's sharp sense of touch.

Sora: But his fists always attracts the vital spots, and I was only able to intercept them with the Sharingan's perception.

Sora: The snowflake pattern.......

Sora: "Destructive Death: Compass Needle."

Sora: He takes on a stance before he engages in battle and activates it....

Sora: Compass.....? The snowflake pattern........?

Sora: Compass..........

Sora: !?

Sora: Is that it!? The battle spirit that he is always talking about!? This realm in the highest......

Sora: Is it because I can "see" through Akaza's attack by the Sharingan!?

Sora: "The eyes that see all truth of creation without ANY obstructions."

Sora: Now then......Akaza's compass......

Sora: How can even sense an attack from behind and make a counter!

Sora: That's it! Inosuke told me once that when he met for the first time, he felt the immense killing intent from my eyes.

Sora: The Sharingan.

Sora: But as for Akaza, he said that my fighting spirit is like the sky.

Sora: Vast and clear.

Sora: It is because of how I analyze my opponents during battle with the Sharingan.

Sora: I remember that I read a book about a famous swordsman, Miyamoto Musashi.

Sora: His book, the Book of Five Rings.

Sora: Earth, Water, Fire, Wind, and the Void.

Sora: The first four books discussed about sword techniques, Musashi-dono's sword style, the heat of battle, flaws in some of the sword arts......

Sora: But the fifth book, the Book of Void.......

Sora: He mentioned something about the things that cannot be seen by the naked eye.

Sora: He also mentioned something about emptiness or some sorts.....

Sora: I know it was somehow all philosophical but........it may also be a way of life.

Sora: Emptiness, void.......

Sora: Void of emotion.......?

Sora: Tanjuro-san?

Time seems to slow down as his mind flashes back to another distant memory in the past.

The memory is set when he was adopted by the Kamado Family and starting to learn the Hinokami Kagura.

-------------

Flashback. Kamado Residence.

Ever since Sora was adopted to the Kamado Family, and noticing that his second family growing.....

The two eldest sons, Sora and Tanjiro talked to Tanjuro about their concerns for his deteriorating health.

Sora: Dad, I guess you should pass on the Hinokami Kagura to Tanjiro and I.

Tanjiro: Nii-san and I decided to learn it properly so that we can take over if it gets difficult for you.

Tanjuro smiled at his two sons.

Tanjuro: Thank you, Sora, Tanjiro. But in all these years, your father never thought the dance was tough.

Sora: What? I mean, even my biological brother was even struggling to complete the Hinokami Kagura before.

Sora: But how come it's not that difficult for you?

Tanjuro: It's quite mysterious. Back then I learned the dance from your grandfather, I was still younger and healthier than I am now.

Tanjuro: It was so hard that my breaths were reduced to wheezes. That is how you felt when you showed me how much can you dance it, right, Sora?

Sora: Mm.

Tanjuro: So, when you were practicing for the kagura.....what did you two notice?

Sora: Hmm......

Tanjiro: I don't know....

Sora: Movements.......somehow there are a lot of movements that I feel like they are useless.

Tanjuro: Correct. Did you notice that I was correcting you to proper steps and breathing.

Tanjuro: Teaching you how to use your maximum strength with minimal movements.

Tanjuro: When you do these, your mind becomes invisible.

Tanjiro: Invisible?

Tanjuro: You have to find these when you two are starting out. Seek the movements and sensations.

Tanjuro: Sora, you have good eyesight. Tanjiro, you have good sense of smell.

Tanjuro: But, you also have to utilize all your five senses.

Tanjuro: Become aware of your whole body down to the tiniest blood vessel.

Tanjuro: It's difficult at first, feeling that it will take you to your limits.

Tanjuro: But once you remember all of these, you remove the unnecessary parts.

Tanjuro: Keep what are the really important movements and close everything else.

Tanjiro: Close?

Sora: But my nii-san told me that the body and mind should be one.

Tanjuro: He's correct, but it doesn't really mean you have to disconnect your mind from your body.

Tanjuro: In fact, your mind becomes even more focused when it becomes invisible.

Tanjuro: Once your mind becomes invisible, you will be able to reach the Transparent World.

Tanjuro: The "boundary" but you will struggle as your strength will be in it's absolute limit.

Tanjuro: Sora, just like your biological brother, who possessed the blessed eyes, you may enter the world when you put your mind into it.

Tanjuro: For those eyes cannot only see the material world but also the world that only those who made their minds invisible have entered.

Tanjiro: Can I even enter the world even without nii-san's eyes?

Tanjuro: Of course.

Sora: Enter the Transparent World.....?

Tanjuro: Sora, the eyes of your clan are special. I'm sure, you can even enter there.

Tanjuro: I know one from your clan who actually entered that world.

Sora: Really? Who?

Tanjuro: Uchiha Takehiro.

Sora: My biological father? You know him?

Tanjuro: Yes. He's actually my best friend.

Tanjuro told Sora about what he knows of Sora's father, with Tanjiro getting interested about Sora's background.

Time passed by.......

Ten days before Tanjuro died from his disease......

One night on the snowy mountains, Tanjuro called Sora and Tanjiro to accompany him.

News of a bear attacking and eating has reached them and as a warning, they put up some ropes with suzu bells to notify them of the incoming danger.

But this puzzled both Tanjiro and Sora as there is no sound coming from the bells. They still followed Tanjuro nevertheless.

When they arrived......

They saw the bear which is two meters tall. The bear which killed six people.

Tanjuro was only standing in front of the bear, with an axe, and the ropes with the suzu bells as the only thing between them.

Tanjiro and Sora watched from behind the trees. To the point that Sora activates his Sharingan as a defense mechanism.

Tanjuro noticed the bear even if it didn't even touched the bells.

Sora: Dad sensed the bear even if the bells were left untouched.

Tanjiro: Dad......

Sora: Even with a sick body, with only a single axe, he's confronting that huge bear.....

Sora: But I feel no anger coming from Dad.

Tanjiro: What?

The two watched Tanjuro confronting the bear.

Tanjuro: I apologize for your hunger. But if you come any closer, I will not let you off so easily.

Tanjuro: If anything or anyone tries to harm my family, I will show no mercy. It doesn't matter who you are.

Tanjuro: And if you don't heed my warning.

Tanjuro takes on a stance.

Tanjuro: I will take your life.

Sora: What's this? Even with that warning, Dad seems to have no emotions at all.

Sora: Even giving a threat to the bear, he's not angry......? I can't hear any hatred from his voice at all.

Sora: Let alone any other emotions.

While Sora is analyzing Tanjuro and the bear, Tanjiro is shaking. Asking himself on what he can do.

But Sora gently taps his little brother's shoulder, a signal that everything will be fine, a signal of saying, "Calm down, Tanjiro."

A few seconds of silence......

Until the bear roared but in a flash, Tanjuro decapitates the bear with an axe.

For Tanjiro, it was a flash. As for Sora whose Sharingan was still barely used during that time, he noticed that Tanjuro chopped the bear twice in order to cut the head of the beast.

But what amazed Sora the most is that, it was precise and clean. To the point that it only took minimum movements to execute such a feat.

The bear's decapitated head and body thumped on the snowy ground, together with the bells.

Sora: He never wavered. Not even a single hesitation, fear.......as if it was empty.

Sora: Standing there like a plant......

Tanjuro slowly turns around to his sons.

Tanjuro: Did the two of you, see that?

Tanjiro: D-Dad.....

Sora: We saw it.....are you okay?

Tanjuro: I'm fine. Sora, Tanjiro, don't ever forget what you saw.

Sora and Tanjiro embraced their father as an act of relief. After that, they paid respects for the bear's soul.

----------------

Back to the present. 3rd POV.

Sora: Dad.......

Sora: The Transparent World........

Akaza then goes for another attack......

Sora: I know the Sharingan is able to predict movements even from the slightest twitch.

Sora: I saw Dad defeating that bear....with my Sharingan.......

Sora: But what amazed me is that, not even a flinch, not even disgust, was there after that.

Sora: ........

Sora: Void.....emptiness.......making your mind invisible.......

Sora: Inosuke even mentioned before that he can sense murderous intent from his sharp sense of touch.

Sora: But if one were to eliminate that killing intent.......

Sora: You'll become invisible......

Sora: *gasp*

Sora: Invisible mind!

Sora: Dad, Inosuke.........

Sora: Are you saying.......!

Sora calmed himself and the moment he opened his eyes.......

He saw Akaza's internal organs.......

Muscle, blood flow, nerves.......

With the Sharingan's aid, Sora was even able to see even the cells......

Sora: So this is......

Sora: The world that Dad was talking about......

Sora: The world that my two fathers.......was able to see.........