The Slave King opened with a flying slash that whistled through the air with deadly precision, but Gustave deflected it with contemptuous ease.
"I didn't expect the legendary Slave King to be such a skilled swordsman," Gustave remarked with dark amusement.
"What do you think has allowed me to build such a vast underground empire?" the Slave King snarled back. "Weakness?"
Without warning, he unleashed a barrage of flying slashes, each one more powerful than the last. The attacks carved through stone pillars and support beams like paper, leaving deep gouges in the underground complex's floor.
Facing the incoming assault, Gustave raised his hands toward the ceiling. "Dragon de Tonnerre!"
A massive serpent of crackling electricity materialized above him and surged forward to meet the sword techniques head-on.
The collision between lightning and steel created an explosion that shook the entire underground city. The ceiling above the impact point cracked and crumbled, while the floor beneath their feet gave way in a cascade of debris.
"You dare destroy the empire I spent decades building?!" the Slave King roared, his voice echoing through the collapsing tunnels. "Then you'll pay with your freedom! Someone with your power will fetch a fortune at auction!"
He launched himself forward with explosive force, dust clouds billowing in his wake as he closed the distance between them in an instant.
"Let's see if you're strong enough to back up those words!" Gustave replied, electrical energy wreathing his arms as he rushed to meet the attack.
When their powers collided this time, the explosion dwarfed their previous exchange. The entire underground complex shuddered violently, and cracks spread through every remaining wall and support structure.
The tremors from the battle sent the escaping slaves tumbling to the ground, with the most terrified among them cowering and trembling at the display of raw power echoing from below.
It was then that the Slave King's two elite guards finally caught up to their prey, their eyes locked on the three mermaids struggling to keep up with the other escapees.
Hearing the approaching footsteps, the three young women turned to see a massive rhinoceros and a black-fisted fighter bearing down on them with predatory grins.
Having never witnessed such violence before, the mermaids froze in terror, unable to move or even scream.
That's when Po's rotund black and white form dropped from above like a meteor, his massive fist connecting with both guards simultaneously and sending them skidding backward across the broken ground.
"Leave these two to me! Get moving!" Po called over his shoulder, his usually gentle voice now carrying the authority of a true martial arts master.
The three mermaids nodded dumbly, shocked to discover that the adorable, chubby panda possessed such incredible strength. His cuddly appearance had completely masked his true capabilities.
"Go! What are you waiting for?!" Po urged when he noticed they were still standing there in amazement.
"Y-yes!" the mermaids stammered, finally breaking from their stupor and fleeing toward the surface.
Po wasn't worried about defeating two opponents - he was concerned about protecting the civilians from the collateral damage that would result from a serious fight.
The two guards exchanged meaningful glances as they picked themselves up from the rubble. They'd clearly underestimated this opponent, and their casual approach had just cost them the element of surprise.
With silent coordination born from years of partnership, they activated their most devastating combination technique. The attack they unleashed carried enough destructive force to seriously injure even an Admiral candidate.
Unfortunately for them, Po had already surpassed that level entirely. His strength had reached true Admiral class through pure martial arts mastery, without relying on any Devil Fruit enhancement.
Faced with their ultimate attack, Po simply raised his fists and met it head-on.
The moment of impact sent a single step backward through Po's massive frame, but his opponents weren't so fortunate. Both guards went flying through the air, crashing through multiple stone walls before finally coming to rest unconscious in a pile of rubble.
While Po and Gustave had successfully neutralized the major threats within the underground complex, a handful of the Slave King's surface operatives had managed to organize a counterattack against the escaped prisoners.
The freed slaves quickly realized that escaping the Slave King's clutches would be far more difficult than they'd initially hoped.
That's when Tom stepped forward.
From seemingly nowhere, the cartoon cat produced an enormous cannon, followed by a cannonball that appeared to weigh several times more than he did. The slaves watched in fascination as Tom struggled comically with the massive projectile, his legs shaking and sliding as he attempted to load it into the barrel.
After what seemed like an eternity of slapstick effort, Tom finally managed to get the cannonball properly seated. He leaned against the cannon, panting dramatically, while the slaves looked on with a mixture of terror and confusion.
Finally catching his breath, Tom produced a match from thin air and struck it against his own rear end. The flame flickered to life, and he carefully lit the cannon's fuse.
Then, in perfect cartoon fashion, Tom turned around, squeezed his eyes shut, and plugged his ears with his paws just as the cannon roared to life.
The cannonball sailed through the air... and missed its target completely.
The Slave King's men burst into mocking laughter, jeering at Tom's apparent incompetence. But what they failed to notice was the cannonball ricocheting off some invisible obstacle, changing course, and demolishing a building directly above their position.
The collapsing structure buried every single mocker beneath tons of stone and timber.
The freed slaves stared in stunned silence, slowly beginning to understand that Tom's "failures" might be more deliberate than they appeared.
Back at the main battlefield, Gustave and the Slave King had fought their way from the underground complex to the surface. Their battle had reduced most of the subterranean city to rubble, though fortunately Po and Tom had successfully evacuated the majority of the prisoners.
Seeing his life's work reduced to ruins and his "merchandise" escaping to freedom, the Slave King's fury reached a boiling point.
"Damn you to hell! I'll make you my personal slave for eternity!" he screamed, his power surging to levels that seemed impossible for a human being.
The sudden increase in strength caught Gustave off guard. Where before they had been evenly matched, he now found himself being steadily pushed back by increasingly vicious attacks.
Under the Slave King's full assault, Gustave took a devastating blow that sent him flying through the air. Fortunately, Po was there to catch him before he could crash into the rubble.
"Gustave, are you alright?" Po asked with concern, noting the blood trickling from his friend's mouth.
"I'm fine," Gustave replied, though he coughed up another small amount of blood that suggested otherwise.
"Do you need my assistance?" Po asked, reading the deteriorating situation correctly.
"No... this is my opportunity. Take Tom and the prisoners and get them away from here," Gustave insisted, wiping the blood from his lips with determination.
Po hesitated, clearly reluctant to leave his friend to face such a dangerous opponent alone. But Gustave's tone brooked no argument, so the martial arts master retreated to a safe distance while keeping a watchful eye on the battle.
Returning to face his opponent, Gustave raised both hands toward the storm clouds gathering overhead.
"Cent Millions de Volts!"
The sky erupted in response. Countless lightning bolts crashed down like divine judgment, turning the entire area into a hellish landscape of electrical destruction. Every remaining member of the Slave King's organization was instantly vaporized by the overwhelming assault.
The Slave King himself emerged from the electrical storm smoking and singed but still standing, his rage now transcending mere fury and reaching the realm of pure hatred.
"You've destroyed everything... EVERYTHING I built!" he roared, his voice cracking with emotion. "Even if it costs me my life, even if every enemy I've ever made comes for me, I WILL END YOU!"
He charged forward with the desperate strength of a man who had nothing left to lose, gathering every remaining ounce of power for one final, devastating attack.
Gustave met his charge with equal determination, both fighters pouring everything they had into this ultimate collision.
As their powers built toward the breaking point, Gustave felt something deep within his soul crack open like an eggshell. A barrier he'd never even known existed suddenly shattered, and power beyond his wildest dreams flooded through his being.
"Overcharge -Dieu du Tonnerre [Thunder God]!"
A towering giant of pure electrical energy materialized around Gustave, standing over one hundred meters tall with his face clearly visible within the crackling blue form. The Thunder God's hair flowed like liquid lightning, and his eyes blazed with the power of heaven's wrath itself.
The massive figure's appearance sent shockwaves across the entire Sabaody Archipelago, drawing the attention of every powerful individual within its borders.
At the Rip-off Bar, Rayleigh felt the surge of power and immediately began racing toward Island 1 at full speed. When he arrived to see the Thunder God towering over the battlefield, his expression grew thoughtful.
"The young man who was drinking juice at your bar seems to have gotten into quite the fight with the Slave King," Shakky observed, appearing beside him.
Rayleigh simply nodded, saying nothing but clearly impressed by the display of power.
Aboard Bamboo Staff, Jinbe and the three children watched the distant giant with awe and concern.
"That's gotta be Uncle Gustave! Look how amazing he is!" Luffy shouted, pointing excitedly at the Thunder God while bouncing up and down on the ship's deck.
Jinbe remained silent, but his furrowed brow revealed his thoughts: Just how powerful is this man?
Under the stunned gaze of everyone watching, the Thunder God raised one massive fist toward the sky. Destructive energy gathered in his grasp, building to a crescendo that seemed to dwarf the sun itself.
When the punch finally descended, it struck with the force of divine judgment. Light brighter than noon blazed across the archipelago, followed by a thunderous roar that shattered windows for miles around.
A mushroom cloud slowly rose from the impact site, marking the end of the Slave King's reign of terror and the birth of a new legend in the One Piece world.