Collision  

Renji Abarai's senses were overwhelmed. All he could perceive were countless explosions, the air filled with the deafening roar of clashing blades and the blinding light of erupting flames. The figures of Zaraki and Ailin were lost in a blur of motion, too fast for his eyes to follow. All he could see were the circular ripples of condensed air left by sonic booms, the fiery bursts of energy from their collisions, and the terrifying shockwaves that ripped through the battlefield.

They were too strong! Even at this distance from the epicenter, Renji felt the tremors shaking his very core. Any closer, and he might be injured by the mere aftershocks of their battle.

Even Kiganjō, Kusajishi, and Ikkaku had retreated further than their initial positions.

As he felt the ground beneath his feet tremble and heard the thunderous clang of their swords clashing, a memory surfaced from his childhood, before his time at the Shin'ō Academy. He had felt the earth shake then too, a distant rumble echoing through Rukongai. It had been about ten years ago. Could it be that what he and Rukia had experienced back then was the aftermath of a battle between Zaraki and Ailin?

…

Clang!

Their Zanpakutō collided once again, sparks erupting from the point of impact. The shockwave pulverized everything around them. Zaraki's body radiated golden reiatsu, enhancing his attack, defense, and speed. Yet, Ailin, seemingly unfazed, effortlessly countered every strike, his reiatsu barely even flickering.

No matter how ferociously Zaraki attacked, he couldn't gain an inch.

Boom!

Their blades met, but there was nothing left to destroy in their immediate vicinity. Beneath their feet, a massive crater, hundreds of meters deep, had formed. The surrounding landscape resembled the desolate, yellow sands of Hueco Mundo.

Clang! Clang! Clang! Clang!

Their Zanpakutō locked in a brief stalemate, a shower of sparks flying as their immense strength pressed against each other.

"I must say, Zaraki, you've made considerable progress these past ten years."

"Hah? Isn't that obvious?" Zaraki grinned savagely. "To get stronger, I haven't slacked off for decades. I've been training with that woman, learning so-called 'swordsmanship.'"

"Oh?" Ailin's eyes widened in surprise. It explained why he had noticed such a dramatic improvement in Zaraki's swordsmanship during their last two battles. He had assumed Zaraki's "wild instincts" had awakened, allowing him to learn the name of his Zanpakutō.

But it seemed Zaraki had been training with Retsu Unohana, the captain of the 4th Division, all this time.

Perhaps it also shed light on why Captain-Commander Yamamoto had refrained from teaching Zaraki swordsmanship. With his immense reiatsu and a mastery of swordsmanship that could fully utilize that power, he might even become a threat to Yamamoto.

Of course, this wouldn't be a concern if Yamamoto were still in his prime. Unfortunately, at over three thousand years old, his body was no longer as youthful as it once had been. While his mastery of reiatsu and combat had grown, his stamina had drastically declined.

He could still effortlessly dispatch opponents who couldn't withstand his initial onslaught. But if a battle dragged on, his stamina would become his greatest weakness.

Undeniably, Ryūjin Jakka, the oldest and strongest fire-type Zanpakutō, was a formidable weapon in Yamamoto's hands. Its Bankai, Zanka no Tachi, possessed a temperature of 15 million degrees Celsius, capable of incinerating the world. In the realm of Shinigami, battles were ultimately a clash of reiatsu. A vast difference in power could render even the most potent abilities useless. Yamamoto's reiatsu was already at the pinnacle for a Shinigami. With Bankai, he could even surpass that limit, reaching a higher dimension of power. Even Ichibei Hyōsube would be affected by the intense heat of Ryūjin Jakka.

However, if an opponent's reiatsu was on par with his, they could potentially use their own reiatsu to shield themselves from Ryūjin Jakka's heat.

As long as they avoided a direct hit, the primary consequence would be a rapid depletion of their reiatsu. If Zaraki's full reiatsu were to awaken, and it proved comparable to Yamamoto's, and if he chose to avoid direct confrontation with Zanka no Tachi, Yamamoto wouldn't stand a chance.

Of course, such a scenario was highly unlikely. If Zaraki ever started avoiding battles, he would cease to be Zaraki. A clash between them would likely result in mutual destruction, Zaraki dying and Yamamoto severely wounded.

"No wonder…"

"But Zaraki, have you yet to discover your greatest weakness?"

"Hah? My weakness?" Zaraki roared with laughter. "After all these years, I already know my greatest weakness. I'm simply not strong enough!"

With a shout, he unleashed another burst of reiatsu, amplifying his power even further as he attempted to push his Zanpakutō through Ailin's defenses. But to his surprise, even with his full reiatsu unleashed, he still couldn't gain an inch.

"No…" Ailin shook his head. "It's not that you're not strong enough. It's that I'm too strong. That's why you have the misconception that you're weak. Even if I were fighting someone else, their performance wouldn't be significantly better than yours…

Zaraki, haven't you noticed? Powerful reiatsu can provide a massive boost. Strength, speed, defense, reaction time… Everything is enhanced."

The difference between a Shinigami releasing their reiatsu and holding it back was comparable to the difference between Shikai and their base form.

"But Zaraki, with your current level of reiatsu, you should be able to sense it, shouldn't you?"

Despite Zaraki's unleashed reiatsu, Ailin remained unmoved. He didn't even break a sweat. His voice was calm and steady.

"Hah? What are you talking about?" Compared to Ailin's effortless control, Zaraki was practically fueled entirely by his battle lust.

"Oh, so your reiatsu still needs some work." Ailin's eyes flickered with disappointment. While Retsu Unohana was merely a persona created by Yachiru Unohana, who could say for sure that it wasn't a part of her true nature?

The bloodthirsty killer, reveling in the thrill of combat, was Yachiru Unohana. The compassionate, nurturing woman, like an older sister, was also Yachiru Unohana.

If possible, Ailin didn't want Zaraki to break through his reiatsu limitations and learn the name of his Zanpakutō at the cost of Unohana's life.

Perhaps his own reiatsu was simply too overwhelming. What seemed like a massive power boost to others, that final step towards the limits of a Shinigami, felt insignificant to him. It had skewed his perception of Zaraki's reiatsu strength.

However, he had never expected Unohana to successfully awaken Zaraki in a single night, while he had been trying to nudge Zaraki towards that breakthrough for decades, with no success.

"I might be telling you this a bit early, but since we're already on the subject, I'll explain it clearly. Otherwise, I might forget next time and assume I've already told you."

"Zaraki, reiatsu is a Shinigami's most valuable asset. A Shinigami's battle is a battle of reiatsu. Powerful reiatsu provides a massive boost, allowing a Shinigami to overcome and even negate certain abilities. However, when one's reiatsu reaches a certain level, their body must be able to withstand it. Otherwise, their own overwhelming reiatsu will destroy them before it can harm their opponent."

Ailin believed that the title of "Kenpachi," the strongest Shinigami, was a fitting one for Zaraki. Even compared to Genryūsai Shigekuni Yamamoto, Zaraki was no less formidable. His physical strength was unmatched among all Shinigami. No one doubted that.

However, even his impressive physique paled in comparison to his reiatsu. During the Thousand-Year Blood War, Zaraki survived being launched into the vacuum of space by Gremmy Thoumeaux, the strongest Sternritter, whose Schrift was "The Visionary." To fight Zaraki on equal footing, Gremmy imagined himself possessing the same monstrous strength. However, the moment he achieved that "vision," Zaraki's overwhelming reiatsu tore his body apart, leaving only his brain intact.

Yamamoto was incredibly strong, the strongest Shinigami in a millennium. But even he, with his immense power, could still maintain his physical form after activating Bankai.

And Kenpachi? Releasing his Shikai was manageable. His body could withstand the surge of reiatsu. However, his Bankai unleashed such overwhelming power that every swing of his blade threatened to tear him apart.

It suggested that Zaraki's reiatsu, in his Bankai state, surpassed even Yamamoto's.

Of course, perhaps Yamamoto, through centuries of training, had managed to tame the wild reiatsu of Ryūjin Jakka, minimizing the strain on his body. Zaraki, on the other hand, was experiencing his Bankai for the first time, his control lacking.