Two pillars unbroken: How Rengoku and Tengen could have changed the course of the Infinity Castle arc

If there’s one thing that keeps me up at night when I think about Demon Slayer: Kimetsu no Yaiba, it’s imagining what the Infinity Castle arc could have been like if Rengoku and Tengen were part of the final charge.

Every time I rewatch the Entertainment District or Mugen Train, I can’t help but picture these two veterans, the fiery, unrelenting Rengoku and the cunning, stealth-driven Tengen, navigating the ever-shifting halls of Nakime’s twisted domain.

Rengoku would have been locked in from the moment he set foot inside, while Tengen’s shinobi instincts would’ve made him a nightmare for demons relying on tricks and ambushes. And honestly, the thought of it changes everything.

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Rengoku’s flame in the darkness

Rengoku’s death on the Mugen Train was both beautiful and devastating, but strategically, it left the Demon Slayer Corps without its most aggressive frontline fighter. In the Infinity Castle arc, where Upper Moons lurked in every corner, that loss was enormous. Rengoku’s fighting spirit alone could’ve turned the tide in several of those confrontations.

Take the battle against Kokushibo, for example. The Upper Rank One wasn’t just powerful; he was composed, calculating, and obsessed with perfection. Facing him were Muichiro, Genya, Sanemi, and Gyomei, all formidable, yet constantly pushed to their limits. Imagine Rengoku in that mix, flame-breathing clashing against Moon Breathing. His intensity would’ve drawn Kokushibo’s attention immediately.

Not only because of his strength, but because Flame Breathing traces its lineage back to Sun Breathing, the very technique Kokushibo despised for reminding him of Yoriichi. Rengoku’s mere presence might’ve triggered old memories and emotional instability in Kokushibo, forcing him off balance in a way few others could.

Rengoku also carried something the Corps desperately needed during that arc: unshakable morale. Every Hashira fought with their own demons, their own trauma, but Rengoku was different. He embodied faith and encouragement, something that could have reignited hope in the others during that claustrophobic, endless battle inside Nakime’s maze.

And let’s be honest, a marked Rengoku would’ve been an absolute monster. He nearly matched Akaza’s raw speed and power without a mark or Hashira training arc buff. Give him the mark, a more seasoned combat perspective, and that relentless drive to protect the next generation, I genuinely believe he could have helped tip the scales in multiple fights, especially against Dōma or even Kokushibo.

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Tengen’s flamboyant stealth: The perfect counter to Nakime

Then there’s Tengen Uzui, the former Sound Hashira, whose strength was often overshadowed by his flashiness. Many forget that beneath his flamboyant personality lies a lethal assassin trained in the art of infiltration, diversion, and silent killing.

In the Infinity Castle arc, that skillset would’ve been invaluable. Nakime’s control over the castle depended heavily on her perception, her ability to see, sense, and reposition enemies. Tengen, whose entire fighting style is built on confusing opponents and masking his presence, would’ve been her worst nightmare.

I can picture it vividly: as the walls twist and floors shift, Tengen uses his Musical Score technique to analyze the sound patterns of the castle itself, identifying where Nakime’s biwa notes are coming from. He’d slip through the corridors without disturbing the air, something Inosuke himself noted during the Entertainment District arc, meaning even Nakime’s all-seeing eyes might fail to track him.

He wouldn’t go in guns blazing; he’d go in quiet, maybe even accompanied by his trusty muscle mice, setting traps, distracting Nakime, and relaying her position to the other Hashira. If he managed to reach her room, even briefly interrupting her performance on the biwa, that alone could have bought the Demon Slayers the precious seconds they needed to regroup.

And while Tengen isn’t as rawly powerful as someone like Gyomei or Sanemi, his battlefield intelligence is on another level. Remember, he fought an Upper Rank Six that had two bodies, Gyutaro and Daki, and still managed to come out alive despite losing an arm and an eye. The Infinity Castle was built for chaos, and chaos is exactly where Tengen thrives.

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What their presence would’ve meant in the Infinity Castle arc

The Infinity Castle arc was a slaughterhouse for both sides. Hashira fell, young slayers died, and Muzan’s fury felt unstoppable. But with Rengoku and Tengen in play, the structure of that war changes dramatically.

If Rengoku had taken part in the Kokushibo fight in the Infinity Castle arc, Muichiro might have lived longer. Rengoku’s defensive prowess could’ve shielded Genya during his crucial moments of transformation.

If Tengen had infiltrated Nakime’s chamber earlier, the entire battlefield in the Infinity Castle arc might’ve stabilized, the disorienting layout could’ve been neutralized, allowing coordinated Hashira assaults rather than scattered duels.

Even Shinobu’s death against Doma could’ve gone differently. With Tengen’s precision bombs countering Doma’s ice constructs and Rengoku’s flames melting his ice-based defense, Doma’s battlefield control would’ve been severely limited. It wouldn’t have been an easy win, but it would’ve been a fairer fight.

And if both had reached Muzan’s final confrontation, their experience as field leaders, the ability to coordinate others mid-battle, might’ve reduced the massive casualty count that took place in their absence in the Infinity Castle arc.