When Karl sat on the giant flounder, he was actually releasing his Observation Haki, covering the entire Fish-Man Island.
He had been waiting for the weapon thrown by Vander Decken IX from outside Fish-Man Island.
As soon as the weapon approached Fish-Man Island, Karl would instantly detect it.
Then, based on the weapon's trajectory, he could lock onto the general direction of Vander Decken IX's location.
Therefore...
While talking with Shirahoshi, Karl saw the weapon flying from outside Fish-Man Island.
After leaving a message for Shirahoshi, Karl activated his electric field, used teleportation to disappear from the Hard-Shell Tower, and appeared at the edge of the area his electric field could currently cover.
Then another teleportation...
His figure transforming into an electric current, Karl left Ryugu Palace in just a few teleports.
After a dozen more teleports, he was outside of Fish-Man Island.
Appearing outside Fish-Man Island, Karl's monstrous physique ignored the deep-sea pressure at 10,000 meters, and he continued to teleport in the direction the weapon had flown from.
His unique perception, capable of covering a large island, allowed him to still see Fish-Man Island even as he teleported further and further away from it.
After a dozen more teleports, Karl's perception could still see Fish-Man Island.
But the deep-sea area he was in had lost all sunlight, becoming completely dark.
However, this kind of darkness had no effect on Karl.
On the contrary, the darkness was more advantageous for his perception.
Except for super-monsters like Karl and Kaido, ninety-nine percent of ability users on the sea feared the ocean.
Even fish-men and merfolk, who are masters of the sea, would become unable to move if they became Devil Fruit users and fell into the water; at most, they could only preserve their lives.
Therefore...
To escape the pursuit from Fish-Man Island, Vander Decken IX never dared to get close to the island, let alone enter the sunlit deep-sea region.
He hid on a ghost ship.
Because he was a Devil Fruit user, he was afraid that he might accidentally break the bubble on his hand, causing his ability to track Shirahoshi to fail, so Vander Decken IX never dared to be in complete darkness.
The dark place he was in would always have lights on, or he would be carrying a lamp.
A single lamp, in the vast darkness of the deep sea, was not even as bright as a firefly's glow.
But to Karl, that point of light was as glaring as the sun.
"Finally found you..."
After hundreds of long-distance teleports, completely distancing himself from Fish-Man Island to the point where even his unique perception could no longer see the sunlit deep-sea region, the corner of Karl's mouth suddenly curled up slightly.
After a few more teleports, Karl appeared in a deep-sea area.
Below, there was a ghost ship that blended into the darkness, so well that even creatures with powerful eyesight would have difficulty spotting its existence.
Inside the ghost ship, there was a room flickering with an incredibly dim light.
"El Thor!"
Karl did not teleport inside the ghost ship.
Nor did he bother to have a conversation with Vander Decken IX before subduing him.
Directly withstanding the pressure of the 10,000-meter deep sea, he converted his stamina into blue-white lightning. Then, his wrist and part of his arm detached from his body, transforming into a massive pillar of lightning that roared out.
SWOOSH—BOOM!
The dazzling blue-white light, like an indestructible blade, tore through the darkness that shrouded the deep sea.
Everything within a thousand-meter radius was exposed under the blinding light that was like the sun.
When the pillar of lightning, powerful enough to destroy a large sailing ship, landed on the ghost ship exposed in the blue-white light.
A massive explosion, like a torpedo, detonated in the deep sea.
A giant wave, like a tornado, along with the lightning pillar, pulverized the already dilapidated ghost ship.
Countless fragments floated in the deep sea.
As the blue-white electric light from the lightning pillar slowly dissipated.
Before darkness could once again envelop the area, Karl teleported into the water tornado, grabbed the corpse that was being swept away, and teleported away, disappearing from sight.
Yes, a corpse!
Believing that the water pressure might affect his power, Karl had first used Life Return before using El Thor.
To be hit by a charged El Thor and not be instantly cooked was a testament to Vander Decken IX having a very strong physique.
One must know, seawater conducts electricity!
Just like fighting in Gran Tesoro, when Karl fights in the sea, his strength is not weakened; on the contrary, it is multiplied.
This was the reason he confirmed his guess and didn't give the Rumble-Rumble Fruit to Nami, instead letting the little cute dragon eat it.
He clearly had the black stone coffin that could steal Devil Fruits...
He clearly knew of many powerful Devil Fruits, so why was Karl only fond of the Rumble-Rumble Fruit?
Because the strongest Logia, the Rumble-Rumble Fruit with no weaknesses, would give him an advantage in the sea that was no less than that of the Ice-Ice Fruit.
If those powerful non-ability users tried to use the advantage of a sea battle to pull Karl into the ocean, then they would be the unlucky ones.
Because if he fell into the sea, not only would Karl's combat power multiply, but his stamina consumption would also be pitifully low.
Of course, not being afraid of seawater was one of Karl's trump cards.
All enemies who saw this trump card had to die.
Carrying Vander Decken IX's body, Karl took about ten minutes to finally return to Fish-Man Island.
"Lord... Karl..."
When Karl used electric heat to dry the seawater from his body and appeared before Neptune with Vander Decken IX's corpse after a series of teleports.
Looking at the corpse in Karl's hand, Neptune's eyes widened in utter shock, "Vander Decken IX!"
How long had it been?
Didn't he go to see his precious daughter Shirahoshi?
Why did he suddenly appear here with the corpse of Vander Decken IX, a man they couldn't find for six years?
"It seems I didn't find the wrong person."
Tossing Vander Decken IX's corpse to the ground, Karl looked at Neptune, who seemed to have countless question marks popping up above his head, and smiled, "This guy suddenly threw a weapon. I followed the weapon's trajectory, used my Rumble-Rumble Fruit ability and bubble coral to chase after him, and finally found him in the deep-sea zone."
"Because we were fighting in the deep sea, as a Devil Fruit user myself, I couldn't afford to hold back, so I accidentally killed him."
"Hiss..."
Looking at Karl's nonchalant expression, Neptune couldn't help but gasp.
He had a new understanding of the kind of monster that was a ruler of the seas.
...Is this the terror of the strongest Logia user?
"King Neptune, since this guy has been brought to justice, shouldn't Princess Shirahoshi be let out of that prison-like place?"
Seeing Neptune, who was too shocked to speak, Karl reminded him.
"Right... right!"
Neptune replied blankly, clearly not having snapped back to his senses yet.
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