Though his Devil Fruit powers were suppressed, Kaido's innate physical constitution was monstrously powerful. If it weren't, how could he have earned a seat on Rocks' ship years ago? Simply by looking ferocious?
The power of the Dark-Dark Fruit, ultimately, is not Seastone; it does not drain an ability user's physical strength to the point of total limpness. Thus, after being forced back into his human form, Kaido immediately tried to exert his raw strength to break free from Diarmuid's iron grip.
He actually had leverage to borrow: the immense repulsive force of Shinra Tensei. He intended to use Diarmuid's own power against him, letting the repulsion blast him away from the Admiral's grasp.
Diarmuid felt Kaido's struggle instantly.
Meanwhile, Charlotte Linlin found her Cognac blade unable to descend an inch against the repulsive wall, even as thick dark mist began surging toward her. Seeing Kaido's pathetic state, Linlin knew these dark elements were dangerous. "Better you than me," she thought. She pivoted mid-air, swinging her blade to cleave through the encroaching darkness before using the powerful repulsion to propel herself backward and escape the gloom.
Without Big Mom to distract him, Diarmuid instantly deactivated the repulsion. Kaido, caught off guard by the sudden disappearance of the force he was resisting, stumbled forward. A massive fist met his face a second later.
"So you're 'Hundred Beasts,' huh?" Diarmuid's first punch landed squarely, the shockwave rattling Kaido's brain. [1]
Before Kaido could speak, Diarmuid followed up with a barrage of three more heavy strikes.
"The 'Strongest Creature,' right?"
"A 'Yonko,' right?"
"Think you're 'Invincible,' right?"
"Love 'Suicide,' right?"
With every question, Diarmuid's fist fell like a falling mountain. Each hit was impossibly heavy. In a matter of seconds, Kaido's face was beaten into a purple, swollen mess.
Without the mythical Zoan's enhancement and healing, Kaido's durability, while still immense, was missing its crucial support. Diarmuid's strikes shattered his defenses with ease.
Blood sprayed from his nose as the relentless assault left Kaido dazed. Who am I? Where am I? What am I doing? As these existential questions flashed through his mind, Kaido regained a flicker of consciousness and screamed, "Linlin, save—"
The word was cut short as another punch pulverized his lips and loosened several teeth. Blood masked his face, making him look utterly wretched.
Watching from a distance, Charlotte Linlin felt a chill of horror. How is this man so brutal? He had literally beaten Kaido to the point of crying for help!
Despite her fear of Diarmuid's vicious methods and her wariness of the dark power, she knew if she didn't save Kaido now, she would be next. Steel entered her eyes. She lunged forward, Cognac wreathed in Prometheus's solar fire once more, swinging down at Diarmuid.
Diarmuid frowned. Shinra Tensei erupted again, the supreme repulsion expanding outward. Linlin's assault was halted mid-air. Then, the dark mist surged along the path of the repulsion, snaking toward her like a predatory beast.
Linlin snarled in fury, her momentum breaking as she hacked at the darkness to clear a path, once again choosing to retreat rather than touch the void.
Diarmuid didn't bother chasing her. The moment she backed off, he turned and planted another fist into Kaido's face.
Kaido: "???" Are you kidding me!?
"Calling for backup, are we!?" Diarmuid growled, his punch nearly bursting Kaido's eyeball.
"LINLIN!!!" Kaido wailed again.
Currently, Diarmuid was using the Dark-Dark Fruit to nullify Kaido's powers, while the darkness itself bound Kaido's movements and invisible gravity weighed him down. To be honest, without his Zoan abilities, Kaido truly couldn't break free!
Diarmuid was now fully realizing just how broken the Dark-Dark Fruit was when dealing with ability users. If the original story had played out and Teach had consumed the Dark-Dark Fruit before stealing Newgate's Tremor-Tremor Fruit, Diarmuid doubted even he could have stood against him.
One hand with the Dark-Dark Fruit to suppress the foe, and the other with the power of tremors to hammer them to death. Any ability user caught by Teach would essentially be looking at a death sentence. Diarmuid calculated that, with a successful surprise grab, Teach could have taken down top-tier pirates like Kaido or Big Mom without much difficulty.
As he had arrogantly declared at Marineford: "This era belongs to Blackbeard! I am now invincible!"
Perhaps the only ones who could handle such a threat were Garp, Zephyr, or Shanks—men who didn't rely on Devil Fruits at all and reached the peak through pure physical prowess.
But Kaido was a fruit user, and right now, his life was in Diarmuid's hands!
"Sa—" Another punch. Two of Kaido's front teeth flew out.
Every time he called for help, Big Mom would charge in, only to be forced back by the push-and-pull of Diarmuid's techniques. It wasn't that she couldn't break through; if she were ruthless enough, she could endure the repulsion and push through with a heavy strike. But she was terrified of the dark element that emerged every time she moved. She saw Kaido trapped in it, being used as a punching bag... if she got caught too...
As Linlin hesitated, she suddenly noticed something. No matter how much Diarmuid pummeled Kaido, the hand gripping Kaido's wrist never moved. It stayed clamped tight, never letting go.
This observation struck her mind like a lightning bolt. What qualities do top-tier powerhouses need? Strength? Reflexes? Yes, but those at the very pinnacle share a common trait: an incredibly flexible mind and a sharp eye that can find a way out of a dead end.
"You're bluffing me!?" A look of grim resolve crossed Linlin's face. Cognac ignited with fresh flames as she surged forward with everything she had.
Diarmuid stuck to the script, unleashing Shinra Tensei to repel her while the dark mist coiled toward her. But this time, unlike before, Linlin ignored the darkness entirely. She gritted her teeth, braced her body against the repulsive force, and swung her blade straight at Diarmuid's head.
The dark mist clung to her, but just as she suspected, it only hampered her speed, and it didn't nullify her soul-based powers! Emboldened, she drove her blade down.
Diarmuid's eyes flashed with a hint of regret. He slid backward, his figure melting into the surrounding gloom as if swallowed by the abyss. Linlin's blade sliced through empty air, hitting nothing.
Kaido, finally released, scrambled away from that nightmare of a position.
"His new ability... it can suppress my fruit powers," Kaido panted immediately.
Linlin nodded, having already surmised as much. "If we face him one-on-one, we have zero chance because of that Dark-Dark Fruit. However, I found a weakness!"
"What is it?" Kaido's eyes lit up.
"Diarmuid! Your Dark-Dark Fruit power can only seal one person at a time, and you must maintain physical contact to do it, right!?" Linlin cackled triumphantly.
This was the flaw she had spotted, and her gambit had just proven it!
In the darkness, Diarmuid's figure slowly rose, as if surfacing from deep water. A smile of genuine appreciation played on his lips as he looked at Big Mom. "I thought you were just an old hag living on past glory, but you actually have some brains. I suppose you didn't survive on these seas for decades just by being big."
Though he admired her analysis, Linlin wasn't entirely correct. The ability to suppress fruits depends on a comparison of strength. Against people like Marco or Katakuri, Diarmuid didn't need contact; a mere look or a distant shroud of darkness could seal them. He could even seal multiple targets as he had neutralized Marco and Katakuri simultaneously.
But against monsters like Kaido or Big Mom, he had to go all-out. The Dark-Dark Fruit, after all, was still just a Devil Fruit. He could absorb attacks with darkness, but to completely seal Kaido's core power, he needed physical contact. Furthermore, that contact had to be continuous, or the target would regain their freedom.
These were the findings Diarmuid had established through experiments with Sakazuki back at Marine HQ. He could seal a Vice Admiral with ease, but to seal the likes of Sakazuki or Borsalino, he had to take them one at a time.
Diarmuid had some gripes with this limitation, but seeing as Teach had faced the same issue when fighting Ace in the original story, he accepted it. The solution was simple: get stronger. If the day came where the gap between Diarmuid and Kaido was as wide as the gap between him and Katakuri, a mere glance would suffice to shut the dragon down.
Hearing Diarmuid's words, Linlin grinned wickedly and turned to Kaido. "Keep your attacks synchronized. If we strike together, he won't be able to lock us both down!"
Kaido nodded grimly. "Today, he dies!"
One gripped his mace, the other her sword. Their auras began to harmonize, a development that genuinely surprised Diarmuid. Their synergy was far beyond his expectations. He had felt it during their Hakai strike, but now, with their spirits linked as if they were two bodies sharing one mind, Diarmuid was certain.
These two definitely had a thing back in the day! I bet Kaido is the secret father of at least one of those kids in the Big Mom Pirates!
The air grew stagnant as the standoff intensified. Just as Diarmuid prepared to break the deadlock, Kaido and Linlin moved simultaneously without a single word, as if by a pre-arranged signal.
They moved in perfect lockstep, maintaining identical speeds and rhythms as they charged. One from the left with a mace, one from the right with a blade, closing off every possible path of retreat.
Diarmuid smirked. The darkness behind him surged, swallowing him whole.
In the next instant, Kaido and Linlin's attacks struck nothing but void. They realized immediately that Diarmuid's presence had vanished from this dark space. Before they could process this, they noticed the dark arena, this "Abyssal Arena," was rapidly shrinking!
At this moment, even they weren't brainless enough to miss the truth.
"COWARD!!!" Big Mom screamed.
This wasn't an arena! It was a trap!
[1] In case you don't know about the Hundred Beast-thing with Kaido, here is an explanation from wiki:
The "hyakujū" (百獣?) part of Kaidou's epithet is an idiomatic noun meaning "all kinds of beasts/animals" (referring to the animal kingdom), combining the kanji for "hundred" (百?) and "beast" (獣?); it is usually found in the Japanese title attributed to the lion, "hyakujū no ō" (百獣の王?), meaning "king of beasts". With Kaidou's title an obvious derivation ("Kaidou" replacing "king"), it can be translated as "Kaidou of the Beasts". In the VIZ manga translation, as well as the subs and dub of the anime, Kaidou is instead called "Kaidou, the King of the Beasts" to make the connection more obvious.