Let's rewind just a bit. In truth, Sukuna wasn't forced into the Infinity Castle by Hantengu and Gyokko. Those two didn't have the strength for that. If they had, they wouldn't have been so easily killed by Sukuna's First Form: Shiranui.
After landing, Sukuna noticed the swordsmen around him being carried away by the wooden doors constructed by this Blood Demon Art.
He stroked his chin and thought for a moment. In the end, he chose to leap into the Infinity Castle himself, curious to see what other tricks Muzan had up his sleeve.
Having killed three Upper Ranks in a row, Sukuna's expectations for Upper Rank strength had already dropped considerably. Otherwise, he would have simply waited outside for Muzan to send someone or come himself.
Falling rapidly through the Infinity Castle, Sukuna had to admit it looked a lot like a Domain. If he didn't know that Domains didn't exist in this world, he might have actually mistaken the Infinity Castle for an innate Domain.
After landing somewhere in the Infinity Castle, Sukuna was observing the space and wondering what caused the different gravitational directions when he noticed the ground beneath his feet moving rapidly toward some destination.
Sukuna simply leaped calmly away from the shifting section of floor.
But when he landed elsewhere, his entire surrounding area began moving quickly again. Sukuna simply fired off a slash and split it cleanly in two.
He used this as a warning to whoever was controlling the space—don't get things twisted. They were the challengers. They should come to him, not the other way around.
After this happened a few times, Nakime finally gave up trying to move Sukuna and instead sent out Kokushibo and Doma.
So before long, Sukuna saw Kokushibo and Doma appear almost simultaneously, riding the fast-moving floor sections to stop not far ahead of him.
Compared to Doma, with his frivolous expression and "Upper Moon Two" written in his eyes, Sukuna was more interested in Kokushibo—the swordsman with three eyes on each side. He could tell by instinct alone that Kokushibo was different from the other demons. And stronger than Akaza.
Sukuna stroked his chin and smirked. "One, two, three, four, five... Isn't there a six?"
Doma was also sizing up Sukuna. He could smell the familiar scents of Hantengu, Gyokko, and Akaza on him.
This was the "human" who had just killed Hantengu, Gyokko, and Akaza in rapid succession?
Kokushibo, too, was observing Sukuna. And on his ears, he saw those familiar earrings.
Doma smiled and answered. "If you're asking about Upper Moon Six, I don't think he'll be appearing."
Sukuna waved a hand dismissively. "Trash like that doesn't matter. Just come at me together. If I kill you, Muzan will show up, right?"
At this, Doma clapped his hands. "Funny you should say that. We happen to be under orders from Lord Muzan to bring you before him. How about you just come quietly?"
"Are you an idiot?" Sukuna replied coldly. "He should be the one coming to me."
Hearing that answer, Doma shrugged helplessly, realizing there was no chance of peacefully completing Muzan's task. He turned to look toward Kokushibo.
Neither Kokushibo nor Doma had any intention of attacking together. Through a wordless exchange, Doma indicated he'd go first. Kokushibo didn't stop him—a silent permission.
Doma began his assault, moving swiftly behind Sukuna while swinging his golden folding fan.
"If that's the case, then—"
But just as Doma thought he had landed his strike, believing Sukuna—who had defeated Akaza—was nothing special, he realized his hand had been severed at some unknown moment.
Doma could still feel the searing pain from the cross-section of his severed arm. His regeneration seemed to have stalled entirely because of that burning sensation.
Doma stared blankly at Sukuna, who had somehow already turned to face him. Sukuna was slowly pulling back from an upward-swinging motion.
What happened?
When did he swing?
That blade's temperature was strange?
Was that heat suppressing his regeneration?
In truth, if Sukuna had wanted to, he could have simply angled his blade and decapitated Doma on the spot, killing him outright.
But that would have been too boring. Far too boring.
"..."
Not far away, Kokushibo watched that single slash in silence. From that one strike, he could already foresee Doma's fate. He also saw that Sukuna had held back.
And Kokushibo had reached this conclusion not by sensing the Red Blade, but simply by witnessing Sukuna's swing.
That was not something Doma could contend with. Which also made clear why Akaza had died.
At the same time, Kokushibo felt a change within himself. His heart pounded faster. The hand resting on his sword trembled faintly. He couldn't tell if it was excitement or fear from Sukuna's overwhelming power...
Sukuna looked contemptuously at the bleeding Doma and said, "Too boring. You know Blood Demon Arts, don't you? Show me your strongest technique."
Doma stared at Sukuna in shock and immediately flared his remaining fan, unleashing his Rime — Water Lily Bodhisattva .
In an instant, a massive ice Bodhisattva materialized before Sukuna.
"Oh? Ice, is it?"
Sukuna looked up with interest at the giant ice Bodhisattva towering over him. In his view, this way of using ice was crude. Constructing such an elaborate form was itself a waste of power. No matter how beautifully carved, it wouldn't improve output by a fraction.
It reminded Sukuna once again of Uraume, who also used ice-based cursed techniques. At least Uraume never bothered with meaningless theatrics.
The ice Bodhisattva raised a hand to its mouth and exhaled a frigid breath toward Sukuna. Sukuna simply raised his blade slowly.
Third form: Blazing Universe
As Sukuna brought his Nichirin Blade down, the flames coiling around the edge became almost tangible, enhanced by the Red Blade. An outsider would be hard-pressed to tell whether this was merely an illusion created by Flame Breathing or if actual blazing fire clung to the blade.
The frozen breath dissolved the moment it touched the flames. The crimson line of fire cut through the ice Bodhisattva like a bold stroke of paint across a canvas.
Sukuna paid no attention to the ice Bodhisattva, which seemed to twist its face in agony as it melted. Instead, he spoke coldly to Doma, who had used that very moment to wrap himself tightly around Sukuna from behind.
"What exactly do you think you're doing?"