The Demon Slayer Corps plunge into Infinity Castle to defeat Muzan. However, the remaining Hashiras and the Demon Slayers who survived Tanjiro's Final Selection are pitted against the remaining members of the Twelve Kizuki first.
Tropes present in Dimensional Infinity Castle Arc:
- Alas, Poor Villain: Kokushibo and Akaza's final moments are notably somber when compared to the other remaining Upper Ranks (Kaigaku and Doma both die without any sympathy in their last moments, and Nakime gets killed without any characterization about her revealed).
- Akaza reawakens his human memories as Hakuji as he recalls how he lost his father, his mentor, and the love of his life when he was preyed upon by Muzan at his lowest. With the help of their souls, he overcomes Muzan and becomes Hakuji, weeping with guilt at having failed to protect Koyuki, who weepingly tells him "Welcome home, my love," just as he forces himself to stop regenerating and disintegrates.
- Kokushibo sees how monstrous in appearance he's become after looking at his reflection, which leads him to realize just how far he's fallen in pursuit of his inferiority complex against Yoriichi, even though his brother never held a grudge against him. This leads him to give up the fight which, combined with how much energy he expended to survive his decapitation, allows Gyomei and Sanemi to finish him off as Kokushibo laments on why he was even born.
- Arc Villain: All of the remaining members of the Twelve Kizuki (Upper Three Akaza, Upper Two Doma, Upper One Kokushibo), as well as their two newest members (Upper Six Kaigaku, Upper Four Nakime) equally share the spotlight as the main opponents to the major Demon Slayer characters.
- Asshole Victim:
- Kaigaku dies in vain for failing to kill Zenitsu, the very person he looked down upon for being "weak." So much for the times he backstabbed a lot of people that tried to do good in his life, including Gyomei, Jigoro, and Zenitsu himself.
- Doma gets his comeuppance by three Demon Slayers he traumatized after years of acting on his sadistic fetish for eating women—including said Demon Slayers' loved ones. To add insult to the injury, his advances toward Shinobu are rejected in the afterlife when he started developing feelings for her.
- But for Me, It Was Tuesday: Doma has killed so many women in his time that he initially doesn't remember Kanae Kocho's death, or even Inosuke's mother's death by his hands. He even literally picks at his brain just to jog his memory.
- Character Death: If Rengoku's death in the Mugen Train arc wasn't sad enough, this one takes it up a notch big time.
- Kagaya sacrifices himself, alongside Amane and their two eldest daughters, in blowing up the mansion as a means to throw Muzan off guard enough for the slayers and Tamayo to ambush him.
- Shinobu's fight against Doma ends with her death, just like her sister Kanae. Except she is absorbed by Doma in the process though, this ended up being a massive Thanatos Gambit to weaken the Demon with the wisteria Shinobu consumed as a means to weaken him, so that her Tsuguko, Kanao, as well as Inosuke, could eventually finish him off.
- Both Genya and Muichiro perish during their fight against Kokushibo. Genya in particular being sliced vertically in half and unable to use his power to regenerate, whereas the Mist Hashira's limbs are cut off and eventually succumbs to his wounds.
- Tamayo ends her centuries'-long life absorbed by her hated nemesis, Muzan, but not before injecting the Demon King with a drug that would severely weaken him prior to his fight against the remaining Demon Slayers.
- Creepy Child: Doma already had sociopathic tendencies even as a human child, complaining about the smell of his parents' corpses instead of showing sadness or horror.
- Dangerous Forbidden Technique: Kokoushibo is a wielder of Moon Breathing, a secret derivate to Sun Breathing that is lost in Demon Slayer lore—likely due to his betrayal of the corps. And when put to practice, it's deadly power is able to give any Demon Slayer a run for their money (even if it is not as strong as Sun Breathing itself).
- Didn't See That Coming:
- Muzan, and the audience, didn't expect Kagaya to suicide-bomb himself and the mansion just to slow Muzan down.
- Kaigaku wasn't expecting Zenitsu to create a Seventh Form of Thunder Breathing that ultimately finished him off.
- Doma gloats that he managed to kill Shinobu by consuming her, only to realize moments later during his fight with Kanao and Inosuke that she consumed plenty of wisteria that he himself consumed through her. It severely weakens him enough for the remaining Demon Slayers to finish him off.
- Evil Counterpart:
- Muzan, the Big Bad immensely powerful King of Demons who is a Bad Boss to his underlings, finally gets the chance to meet Kagaya, the Big Good sickly leader of the Demon Slayer Corps who is a Benevolent Boss to his soldiers. Oh, and they're also distant relatives of each other.
- Kaigaku is basically Zenitsu Agatsuma's worst qualities put together into one pathetic being. Even as a fellow Thunder Breather, their parallels could not have been more opposite: Kaigaku knows all the forms (and is able to customize them with Blood Demon Arts) except for the first (and most important one)—which is the only form mastered by Zenitsu (aside from a custom-made Seventh Form that ultimately finished Kaigaku off).
- Kokushibo can be considered as one to Gyomei. Kokushibo is Muzan's strongest of the Twelve Kizuki as Upper Rank One (and the strongest of all the Demons next to Muzan), paralleling how Gyomei is the strongest of the Hashira (and the entire Demon Slayer Corps as a whole). It is also worth noting that Gyomei is the closest thing to a right-hand man to Kagaya—being the one person he trusts the most, just like how Kokushibo is the right-hand man to Muzan, the latter of which considers the former as a business partner.
- Hate Sink: Even before he became a demon, Kaigaku was a mean-spirited, self-glorified bully who only earns ire in the eyes of everyone he meets, especially after selling out a fellow group of orphans because he couldn't stand the idea of being caught. Unlike most demons shown, he chose to become one solely because he didn't want to die, and he's a spiteful jackass who'll go as far as to mock his former master and fellow students for being weaker than him. Unlike most demons like Daki, Gyutaro, Akaza, and Kokushubo who were driven by tragic circumstances, and even Doma who can be darkly entertaining despite his own heinousness and is a credible foe in his own right, Kaigaku is a pathetic coward who barks more than he needs to chew on and dies an undignified, yet well-deserved death.
- Hero Killer: Muzan and Doma kill Tamayo and Shinobu respectively by absorbing them. Kokushibo ends Muichiro's and Genya's lives during their fight against him.
- Hypocrite: Kaigaku mocks Zenitsu's weakness for being a coward and not showing their master "Jigoro Kuwajima" the proper respect owed to a former Hashira, yet when Kaigaku encounters Kokushibo who is an Upper Rank 1 demon, he cowardly decides to surrender and give up, which means he ends up disgracing their master and causing him to commit Seppuku.
- Karmic Death: For all of Kaigaku's mockery of Zenitsu's weakness, it turns out he is unaware of the seventh Thunder Breathing form that Zenitsu invented and is easily dispatched by it. Furthermore, Kaigaku's final moments are his complaining and ranting about how Zenitsu got the best for him and he dies utterly alone and with no one to care and mourn for him, underscoring how his cowardly nature paid off for him in the end.
- Last-Second Chance: Despite Muzan cursing his ancestors for his disease, Kagaya talks to Muzan amicably and offers him one final moment to reflect upon his actions before it's too late for him. Muzan refuses it and basically tells him as he is immortal, he is free from all consequences of his millennium of rampage. As a result, Kagaya suicide-bombs himself so as to give the Demon Slayers a golden opportunity to bring down Muzan when he has let his guard down.
- New Powers as the Plot Demands:
- Tanjiro learns to access Transparent World, just in time to counter Akaza.
- Zenitsu is able to create a Seventh Form of Thunder Breathing, at just the right time to mark the killing blow to Kaigaku.
- Offended by an Inferior's Success: Kaigaku always resented Zenitsu because he believed that Jigoro always favored him as his disciple despite Kaigaku seeing him as an incompetent failure. When he ends up decapitated by Zenitsu’s Thunder Breathing, Seventh Form: Honoikazuchi no Kami, Kaigaku frustratingly accuses their master of favoring Zenitsu over him to teach him such a technique, only for Zenitsu to reveal that he developed the technique himself. Kaigaku tries to take solace in the fact that Zenitsu will at least die with him, only for Yushiro to deny him the satisfaction by saving Zenitsu and rub salt in the wounds with his "Reason You Suck" Speech, leading Kaigaku to die screaming in anger and frustration.
- O.O.C. Is Serious Business:
- Zenitsu, the Lovable Coward, and who has to fall into sleep just to fight Demons, loses it and consciously battles Kaigaku after hearing what his actions led to for his master, Jigoro.
- Muichiro had no such trouble taking down Upper Moon 5, Gyokko—barraging him with quips to ruin the latter's fragile ego before quickly finishing him off. Upon coming face-to-face with Upper Moon 1, Kokoushibo, he is shaking with fear at his unmatched power—understandably so.
- Pre-Final Boss: All the remaining members of the Twelve Kizuki serve as this to the major Demon Slayers before the entire Demon Slayer Corps face Muzan Kibutsuji himself. Doubles as Climax Boss.
- Zenitsu finally faces off the recently-demonized Kaigaku not only to avenge their master, Jigoro, but also prove himself to be the latter's worthy successor.
- Shinobu, Kanao, and Inosuke are able to find some form of closure by killing off Doma, the Demon responsible for their tragic pasts.
- Tanjiro also gets some form of closure in avenging Kyojuro's death by facing and defeating Akaza (alongside Giyu).
- The combined efforts of Muichiro, the Shinazugawa brothers, and Gyomei lead to the defeat of Kokushibo, Muzan's Dragon and strongest subordinate.
- Obanai and Mitsuri stall Nakime enough for Yushiro to gain control of her, resulting in Muzan being forced to kill the demon—leading to the Infinity Castle's collapse.
- The Reveal:
- Kaigaku became a demon to avoid getting killed, and Zenitsu's master committed Seppuku in shame.
- Doma, the Upper Rank Two, was the Demon not only responsible for the death of Kanae Kocho, but also Inosuke Hashibura's upbringing by boars.
- Kokushibo is not only Yoriichi Tsugikuni's twin brother, but also the distant ancestor to Muichiro and his brother.
- Thanatos Gambit: Shinobu prepared for her eventual encounter with her sister's killer Doma by consuming enough Wisteria toxin to poison and weaken him, knowing that she likely won't win in a straight fight against him and predicting that he will devour her once he wins. This greatly contributes to Kanao and Inosuke's victory, as they manage to stall long enough for the poison to take effect and give them the opportunity to kill the weakened Doma.
- Worthy Opponent:
- Akaza acknowledges that Tanjiro has vastly improved since their last encounter, and that he never seen a Water Hashira this good in fifty years before encountering Giyu. He even offers Giyu to join him as a Demon because of his strength, just as he had tried to do with Kyojuro previously.
- Kokushibo gives this to Sanemi and Gyomei due to them being the first Hashiras to challenge him after 300 years, who don't reciprocate due to him being a Demon responsible for the murder of thousands.
- You Are in Command Now: Kiriya, Kagaya's only son, does this to himself after the latter's death, as a means to continue rallying the Demon Slayers in their ultimate fight against Muzan himself. It seems as if Kagaya had prepared him for just the right moment.
