The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga or anime.

Summary

Upper Moon One, Kokushibo, will battle against several top Hashira, posing a significant threat to the Demon Slayer Corps's chances of defeating Muzan.
Kokushibo's fight is crucial due to his strength and the potentially devastating consequences of losing key Demon Slayer Corps members.
Revealing Kokushibo's secrets could provide valuable context to the millennia-long battle and increase the Corps's chances against Muzan.

Demon Slayer: Infinity Castle movie is all set to feature some of the most intense battles of the series. All of them will have a deep influence on the overall narrative and will set the stage for the culmination of the millennia-long struggle of good and evil.

There has never been an arc in which so many high-stakes battles take place simultaneously. Fans are excited to finally see some of the most powerful demons duke it out with the Hashira. While almost every fight is set to leave fans in awe, one is set to have the most far-reaching consequence. Upper Moon Kokushibo, despite being so feared, does not feature in the Infinity Castle movie trailer, so who is he going to fight, and why is it so crucial for the show's climax? Let's try to understand.

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Who Will Kokushibo Fight in the Infinity Castle Movie?

Upper Moon One is All Set to Fight Three Top Hashira

The Infinity Castle movie trailer gave fans a glimpse of what they could expect from the trilogy. It teased the battle between Tanjiro and Akaza, Doma and Shinobu, and Zenitsu and Kaigaku. Hashira such as Gyomei, Sanemi, Obanai, Muichiro, Mitsuri, and Giyu were also seen, prepared for the battle ahead. But in all this chaos, one prominent character was missing.

I will stop your bleeding. Humans are so fragile. However... If you bleed to death... Or if his lordship doesn't approve of you... And you die... Then death was always your fate. In that case... You were no greater a man than that. Don't you... Agree? Kokushibo to Muichiro

Upper-Rank One Kokushibo, who has made only a few appearances so far, is the strongest demon alive, only second to the Demon King himself. Although the trailer does not reveal anything about Kokushibo, he is all set to fight the Demon Slayer Corps in the Infinity Castle. In fact, he is going to be the biggest threat to the demon slayers as they try to bring down the Twelve Kizuki and corner Muzan.

Following their arrival in the Infinity Castle, the first two demon slayers unfortunate enough to run into him will be Genya and Muichiro. Despite having a Demon Slayer Mark, Kokushibo will make Mist Hashira feel as if he were an amateur swordsman. The haunting reality of fighting someone of his stature will finally begin to dawn on Muichiro.

A split image of Kokushibo And Yoriichi in Demon Slayer

Genya will also not be so lucky since Tokito will barely offer a challenge to Kokushibo. This will leave him open for a one-on-one confrontation. Just when it will seem that things are taking a dark turn, Sanemi and later Gyomei will arrive at the scene as well. But despite single-handedly taking on all these swordsmen, Kokushibo will seem unperturbed. The battle will be intense with the Taisho Era's most talented warriors fighting to bring down a demon who has refined his swordsmanship for over four centuries.

Why is Kokushibo's Battle so Consequential?

Upper Moon One Could Strike a Death Blow to the Hopes of Defeating Muzan

In the Infinity Castle, demons like Doma, Akaza, Kaigaku, and Nakime will also take part in an intense battle. So what makes Kokushibo's fight so consequential in comparison? Perhaps the biggest difference will be the kind of talent that the Demon Slayer Corps will have to put on the line against the Upper Moon One, to even stand a chance. Gyomei and Sanemi are rated as the strongest members of the Demon Slayer Corps from the current era.

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Furthermore, Muichiro Tokito's prodigious talent is so rare that even Hashira are in awe of him. Genya is another great combatant who has evolved himself to fight demons despite not being able to use a breathing style. The Demon Slayer Corps cannot afford to lose these four, as defeat could potentially bring down the chance of victory against Muzan to zero.

What most fans also fail to realize is the kind of secrets Kokushibo has been harbouring for all these centuries. Once revealed, they will add more context to the battle that has been waged for over a millennium. It could eventually benefit the Demon Slayer Corps immensely once Kokushibo is defeated and Muzan Kibutsuji is forced to fight all alone.

However, it remains to be seen how the battle actually unfolds, cause victory does not always deserves to be celebrated, especially if the victors end up suffering too many casualties. So the challenge against Kokushibo is not just to win, but also to retain the core team, since the battle will only be half won, and Muzan will be waiting for them.

Demon Slayer: Infinity Castle trilogy's first installment is scheduled to be released in the United States on September 12, 2025.