Muten Natsu knew that Spirit Ink could never keep up with Ōko Yushima's teleportation, yet he continued expending his power to attack with it. The main reason was Rukia Kuchiki's assistance.
As Rukia Kuchiki had realized in Hueco Mundo, unlike Hyōrinmaru, which exerted control over the outside world, Sode no Shirayuki's power focused more on releasing cold from within.
More precisely, it controlled temperature.
Hyōrinmaru could not only control the weather but also condense water vapor into dragons that twisted and coiled through the air.
By contrast, the three techniques available after Sode no Shirayuki's Shikai all released cold. They created ice rather than controlled it.
But what if she devoted all the power used to create ice entirely to controlling temperature?
The moment this occurred to Rukia Kuchiki, she began experimenting.
The result was an immediate breakthrough in her control over temperature.
After all, both Zanpakutō belonged to the Ice and Snow element, and Tōshirō Hitsugaya was a renowned genius. It was only natural that Rukia Kuchiki had subconsciously imitated the way he used his power.
But their abilities were fundamentally different.
Once she chose a path suited to her own power, Rukia Kuchiki immediately realized that Sode no Shirayuki's cold did not emanate from the blade itself, but from its tip. The blade merely expanded the area affected by the freezing.
The proof lay in its three derivative techniques.
Some no mai, Tsukishiro. By drawing a circle on the ground with the tip of Sode no Shirayuki, everything within the space enclosed by that circle entered its freezing domain.
Tsugi no mai, Hakuren: Hakuren. By thrusting the blade straight ahead, its tip released an avalanche of cold that froze the enemy.
Final Dance: White Dew. An upward slash released cold from the blade's tip and condensed it into a blade of ice. When the Zanpakutō swung downward, the ice blade plunged down with it.
By redirecting all the ice-creating power used in those three techniques into temperature control, Rukia Kuchiki could unleash extreme cold that even she herself could not withstand.
The drawback was that even a slightly forceful movement, let alone an attack, would cause severe damage to her own body.
Yet without creating ice, Rukia Kuchiki had no means of attacking from a distance, leaving Sode no Shirayuki's power with no way to take effect.
Unless she had a suitable medium through which to transmit that terrifying cold.
As it happened, she had exactly that.
Under Muten Natsu's control, Spirit Ink could flow freely.
Combined with the shaping effect of the cold, it could also be given solid form.
Ōko Yushima's teleportation could only take him into spaces that could be forced open. Both the Spirit Ink and the black mist it released could be displaced, but solid Ink Ice could greatly restrict where he was able to teleport.
Once the Spirit Ink had completely bound the enemy, all Rukia Kuchiki had to do was lower the temperature.
The pitch-black Spirit Ink filling the sky froze solid, completely imprisoning Ōko Yushima within it.
A dark mountain seemed to take shape in an instant. The cold radiating from it made the captains' breathing catch, their faces stinging as though scraped by blades.
Muten Natsu dived down and swept the ice-cold Rukia Kuchiki into his arms. The moment his foot struck the ground, the Hell Flames erupted.
Black fire engulfed the mountain of ice.
Then a terrifying force exploded outward, unleashing a howling hurricane across the peak of the Sōkyoku.
When the dust settled, Ōko Yushima stood trembling at the center of a massive crater, his body drenched in blood. The golden light on his forehead flickered unsteadily.
Sumitsukigasa had already snapped in two. He leaned on the broken weapon to support his critically wounded body.
"It's... not over yet..."
"It's already over," a voice said. "Please... don't continue this any longer..."
Ōko Yushima's eyes flew open. He stared at the speaker in disbelief.
"You... How are you... I clearly... I clearly fused with you!"
The figure reflected in his eyes was none other than Nozomi Kujō, whom he had supposedly fused with earlier.
Ōko Yushima nearly collapsed. The moment his bewildered gaze fell on the man beside Nozomi Kujō, it filled with bone-deep hatred.
"Kisuke Urahara! What did you do?"
"I merely made a Reigai."
Kisuke Urahara held the hat Nozomi Kujō had previously been wearing, the same hat Kagerōza Inaba had tossed aside.
"The Modified Soul Kurōdo can absorb another person's Reishi and transform into them."
Ōko Yushima's heart clenched.
The abnormality that had been concealed beneath his immense power suddenly became unmistakably clear, impossible to ignore any longer.
"The one I fused with... was a Reigai you created?!"
"To be precise, it was a Reigai that a Modified Soul had finished using."
Kisuke Urahara revealed the Gikon Pill beneath the hat.
"The hat is the true body. I installed a controller in the hat and a receiver in the Reigai. That allowed Kurōdo to control the Reigai for a short time and within a limited range, even after they were separated."
"I'm almost impressed, ne." Mayuri Kurotsuchi looked at him as though he were an idiot. "You fused with something that had come into contact with Kisuke Urahara without inspecting it thoroughly first?"
"Waaah—!"
Ōko Yushima vomited a great mouthful of blood.
"Aaaaaah!!!"
He could endure hiding in the shadows. He could accept defeat. He did not even fear death.
But he could not accept that Kisuke Urahara had replicated in such a short time the technology he had spent a century developing, then used it to deceive him.
I was first. I was the one who began this research. I was the one who paid the price for it. I was the one who worked to prove the superiority of the Reigai...
Yet Kisuke Urahara had effortlessly shattered everything Ōko Yushima had worked for.
"Ahhh... Ha... Hahahahaha!!! You truly are incredible, Kisuke Urahara!"
Ōko Yushima's laughter turned shrill. His bloodstained gaze swept over everyone present.
His cause was already lost. There was nothing more to say.
"Die, all of you!"
He raised the shattered Sumitsukigasa overhead.
This would be Ōko Yushima's final Hajō Kūri. He would sacrifice everything he had left to destroy those who had destroyed everything he had...
"Urgh!"
The broken blade fell to the ground.
Ōko Yushima stared in shock at his vanishing arm, then forced the corner of his mouth upward.
"Did you foresee this too?"
"If you had stopped in time," Kisuke Urahara said with a sigh, "you could have survived."
"Stopped?"
Ōko Yushima's body began to disintegrate.
"You must be joking."
Nozomi Kujō stared blankly in his direction, watching as the final particle of Reishi vanished into the sky.
Amid a brilliant flash, a small square seal took shape and slowly drifted downward.
Kisuke Urahara lowered the brim of his hat.
"At a time like this, who could possibly laugh?"
As a fellow researcher, Kisuke Urahara could not help feeling conflicted as he witnessed Ōko Yushima's fate.