One Piece: Kaido's adopted son, starts with the Eight Gates Formation.

"Imprisoned in Hell are beings that the successive captains-commanders of the Gotei 13 have tried to defeat but could not destroy. They are not Shinigami, not Hollows, not humans. They are 'concept' level anomalies—each possessing the ability to distort the rules of reality."

Yamamoto lowered his head.

He knew Aizen was telling the truth. The Hell Realm—that was a taboo subject that even he, who had led the Gotei 13 for ten thousand years, had never mentioned.

"Is he stronger than you?" Kaido asked Yamamoto.

Yamamoto looked up at Kaido, a rare expression appearing on his aged face—evasion.

"Some... are more difficult to deal with than I am." This was the most honest answer Yamamoto could give.

Kaido grinned widely.

"Fine! I'm going!"

"Wait a minute." The white-bearded man tapped the ground with the hilt of his naginata. "Let's get to the bottom of this. Aizen, you said the Soul King's right arm is in Hell? How could it be in a place like that?"

Aizen turned to Whitebeard, his attitude becoming much more respectful—he always maintained a superficial respect for true powerhouses.

"The Soul King's right arm—'The Right of Stagnation'—possesses the power to 'freeze all things.' Hundreds of thousands of years ago, after the Zero Division severed it, they discovered that it could not be sealed or destroyed by any means. This is because everything that comes into contact with it is 'frozen,' including time, space, matter, and concepts."

"So they threw it into Hell," Kyoraku continued, his voice low. "Because the 'worms' of Hell—those conceptual anomalies—can devour anything. They had the worms swallow Stagnation Right."

"If it's swallowed, is it still there?" Kaido asked, puzzled.

"It's inside the worm," Aizen said. "The concepts of 'stillness' and 'devouring' counteract each other, creating a distorted balance. The worm can't digest it, and it can't still the worm. They've coexisted like this for hundreds of thousands of years."

Lynn took a sip of her lollipop and tilted her head.

"So I'm going to the underworld, find that worm, and take the Spirit King's right arm out of its belly."

"Simply put, that's how it is." Aizen nodded.

How strong is that worm?

Aizen thought for a moment and gave an evaluation that silenced everyone.

"If it appeared in the present world, the spiritual pressure fluctuations would be enough to cause the collapse of all spiritual particles in the present world. Converted to your system, that's roughly equivalent to Kaido at his peak multiplied by three to five."

Kaido's smile froze for a second, then became even brighter.

"Five times? Great! That's fantastic! I haven't encountered an opponent stronger than myself in ages!"

"Weren't you just severely injured by Yhwach?" Akainu couldn't help but retort.

"That's because he's cheating! He's nowhere near as strong as me!"

Akainu was too lazy to say anything more.

Lynn jumped off Kaido's shoulder and floated in front of Aizen.

Where is the entrance to Hell?

"Directly below the Seireitei," Aizen pointed to the ground, "a place deeper than Avici Hell. From the eighth floor downwards—the ninth, the tenth—a total of eighteen floors. The very bottom floor is the entrance to Hell. Ordinary Shinigami can't enter, and even the Zero Division doesn't have the authority to open it. The only way to open it—"

He looked at Hyosube Ichibei.

"—It is by using the power of the 'True Name' to invoke the name of the Gates of Hell."

Hyosube Ichibei held his lollipop and remained silent for a long time.

"...I do know the true name of the Gates of Hell," he said slowly, "but once the seal on the Hell Realm is broken, what's inside might escape. If those anomalous entities—"

"If they run away, we'll fight them back," Kaido said nonchalantly.

"It's not that simple." Yamamoto spoke, unusually, his voice heavy. "There's more than one 'worm' in hell. I read ancient records—at least seven. Each one corresponds to a distorted world rule. The one that devoured the right side of stagnation is said to be called 'Annihilation.'"

"Extinction?" Lynn didn't feel anything for the name.

"The corresponding rule is 'extinction'," Yamamoto's voice deepened further. "Everything it touches will come to an end. Not death—it's 'extinction.' Completely eliminating existence from the causal level."