Oto and Kogarashi.
The legendary pirate "Golden Lion" Shiki's famous swords—or more accurately, the blades he now used as his feet.
Twenty-two years ago, after the capture of Pirate King Gol D. Roger, Shiki stormed Marine Headquarters alone.
Defeated by Garp and the then-Admiral Sengoku, he was thrown into the underwater hellscape of Impel Down.
No one expected him to sever his own legs two years later.
Using the power of the Float-Float Fruit, he broke out of his cell and became the first escapee in Impel Down's history.
Ever since then, seeking revenge on the East Blue became his burning obsession.
Roger, the only man he ever acknowledged as an equal, was executed in the East Blue—the so-called "Weakest Sea."
Destroying that sea was the necessary first step in his grand plan to dominate the world.
If the Golden Lion had been causing trouble in the New World, Cain would have ignored him.
But today they crossed paths, and Shiki's target happened to be Cain's homeland.
More importantly, Cain wanted those two swords.
Whether to protect his home or simply to steal the blades, Cain was not going to let the Golden Lion walk away.
...
Rumble—!
The floating island high in the sky trembled violently.
Rock layers cracked, and rubble cascaded into the clouds.
Shiki and his ship doctor, Dr. Indigo, stumbled as the ground shook.
"Ah! Is it an earthquake?!" Shiki yelled.
"It must be your ability acting up! You are the one controlling this island!"
"Well, that's true... No, wait. It's someone else."
Shiki scowled, the ship's steering wheel still deeply embedded in his skull.
Using his Float-Float powers, he levitated into the air and flew off the edge of the island to investigate.
Just below the landmass, a massive pink dragon circled the air.
Its orange flame clouds were actively pushing the island upward, causing the massive tremors.
Cain sensed his target approaching.
He stopped pushing the island, swung his long body around, and hovered right in front of Shiki.
He let out a light gust of breath, blowing Shiki's golden hair back.
"Ah! A giant serpent!" Dr Indigo yelled.
Cain's gaze fell on the steering wheel lodged in the man's head and the swords replacing his feet.
Between the old brain injury from the Edd War and twenty years of hiding in the clouds, Shiki had lost his former glory.
He was sluggish and senile.
Once a powerhouse on par with Roger and Whitebeard, he was now just an old man.
That tragic decline gave Cain absolute confidence in this fight.
Cain didn't give Shiki time to think. He summoned a whirlwind, opened his massive jaws and gathered his power.
"Demolition Gust!"
Piracy was all about plunder.
If Cain wanted those two swords, he simply had to take them. There was no need for conversation or negotiation.
Rather than letting this relic destroy the East Blue, it was better to send him right back to Impel Down.
Green wind blades erupted from Cain's mouth like a meteor shower, hurtling straight toward Shiki.
Shiki snapped out of his confusion and swung his legs.
The legendary swords Oto and Kogarashi slashed through the air, sending out shockwaves that barely fended off the onslaught.
"Hey, brat! Are you Kaido? No, the color is wrong. Who the hell are you?!" Shiki yelled.
"I'm just a pirate passing through the East Blue."
Before Shiki could process the answer, a heavy wave of Conqueror's Haki erupted from Cain.
The massive pink dragon charged straight at the old pirate.
The attack caught Shiki completely off guard.
He pushed his Float-Float powers to rapidly shift his position in the air, narrowly dodging the snapping jaws as fury flashed in his eyes.
"The East Blue again! First Garp, then Roger, and now some brat like you dares to get in my way! That sea really needs to be destroyed!" Shiki roared.
"I was going to recruit you because of that power, but you don't know what's good for you. So.. die!"
Shiki raised his right foot high, and the blade Oto unleashed a massive flying slash.
"Lion's Threat: Slash Wave!"
The strike was powerful enough to split the ocean below.
Cain coated his dragon scales in Armament Haki and effortlessly took the hit.
Sensing the feeble impact, Cain sighed. "Golden Lion, you really have grown old. Can you even use Haki anymore?"
His tone carried a genuine hint of pity, but his assault never stopped.
Roar—!
The Azure Dragon Fruit allowed its user to manipulate the weather.
With a single roar, Cain gathered the surrounding atmosphere into a dense, crackling thunderstorm.
His jaws opened wide, condensing the lightning into a blinding beam that shot straight at Shiki.
"An attack of this level? How could it possibly—"
Shiki smirked, preparing to dodge, but the lightning arced wildly.
It latched onto the metal blades of Oto and Kogarashi, coursing directly through his legs and electrocuting his entire body.
"Fool. Don't you know walking on steel swords turns you into a natural lightning rod?"
Cain felt a twinge of guilt beating up a senile old man, but the Golden Lion was a pure scum.
He felt no guilty bullying him!
"Don't get cocky, brat! No matter where you got that fruit, destroying the East Blue is my destiny!" Shiki spat.
He waved his hands, and the vast reserves of seawater stored on the floating island surged outward, sweeping down toward Cain like a tidal wave.
'Not good...'
Cain fired multiple blasts of intense heat from his mouth, directly intercepting the falling seawater.
The collision created a massive explosion of dense steam, completely blinding them both.
'Damn it, I fell for it!' Cain activated his Observation Haki, but he was a second too late.
"Lion's Strike: Thousand Slashes!"
Faster and sharper than before, a dense barrage of flying slashes pierced through the steam.
They rained down on Cain's giant body, the high-pitched sound of slicing air echoing across the sky.
"Sigh. It doesn't even hurt."
Cain glanced down at his massive body.
There were only shallow white scratches on his scales.
Shiki's sword techniques couldn't even penetrate his natural defense, making the Armament Haki he applied earlier completely unnecessary.
Shiki's strength had decayed so badly he was no longer the monster who once stood beside Roger and Whitebeard.
But this was the perfect opportunity to test a specific move.
Pale orange flames seeped from Cain's jaws, slowly coiling around his serpentine form.
High above, Shiki's heart sank.
Even his full-powered Thousand Slashes had failed to draw the brat blood.
'If this brat's defense truly rivaled Kaido's, this fight was already over.'
He finally had to face the bitter reality: his peak was long gone.
The scorching heat burned away the steam.
Shiki stared at the swirling flames wrapping around the dragon, and genuine fear took root in his chest.
He had no choice but to go all in.
Shiki looked at the floating island he had spent twenty years building, steeled his resolve, and swung his legs.
Oto and Kogarashi unleashed two massive shockwaves that cleanly cleaved the entire landmass in half.
Millions of tons of rock and earth fell from the sky, raining down on Cain like a meteor shower.
'Great Dragon Fire Torch... yeah, that was a failure,' Cain thought.
He tried to envelop his entire body in a massive avatar of fire, but the flames remained scattered and chaotic.
Instinctive moves like Boro Breath or lightning were easy to use, but Kaido's ultimate technique required decades of meticulous control.
He couldn't just copy it on his first try.
He dropped the technique as several hill-sized island fragments crashed toward him.
Cain opened his jaws and unleashed a full-powered Boro Breath.
A blazing pillar of fire shot straight into the sky, incinerating the falling rocks, shattered trees, and the mutated beasts hiding among the debris.
Molten lava splashed against his scales, but he barely felt the heat.
Riding the updraft of his own attack, Cain swung his tail and charged straight at Shiki.
Shiki's internal alarms blared.
He tried to use his Float-Float powers to escape, but residual lightning from the storm clouds suddenly sparked across his swords.
The electricity paralyzed his limbs, freezing him mid-air for a crucial fraction of a second.
In the blink of an eye, Cain was right in front of him.
"Golden Lion, you are too old!" Cain roared, his voice echoing like thunder.
"You are a remnant of a bygone era! Your body, your Haki, your ambition—none of it belongs in this age!"
The flames gathered around Cain's head surged violently.
Carrying the momentum of a falling meteor, he slammed his massive skull directly into Shiki!
The impact shattered the air.
Shiki's body was launched backward like a cannonball, crashing directly into the remaining half of the floating island.
The rock layers buckled and exploded, sending massive shockwaves of dust and debris across the sky.
...
Regarding the plot: This is Cain's first act as a pirate, so he goes straight for the attack.
Since Golden Lion's strength has severely declined and there are no other major factions involved right now, there is no need for drawn-out dialogue or elaborate setups.
Striking first to prevent harm and steal the swords is perfectly reasonable here.
(Note on power scaling: Golden Lion's strength is notoriously inconsistent since he was defeated by pre-timeskip Luffy in the movie. Defeating him at Cain's current power level makes sense).
From a writing perspective: The movie timelines are too messy.
For movies like Stampede (Bullet) or Red (Uta), the Straw Hat crew is established, making it impossible to fit them naturally into the early canon timeline without breaking logic.
The only way to include them is to advance the encounters or set them up indirectly.
Uta is a major plot point that will be focus on later in the story.