On a gashed up ship named the Sally Rover a powerful looking man stood upon its prow watching a fight through his spyglass. He is Rox D Xebec and he had long spiky messy dark red hair and wore a coat like a cape over his bare shoulders. He smiled broadly to show off square even teeth, then lowered his spyglass. That smile does not touch his empty wild looking eyes nor does it change his statuesque granite like face, save to make him look more deranged.
"Hey Edward! Get over here!" Xebec commanded in a baritone voice that was both oily and raspy.
The future Emperor of the Sea, Whitebeard, walked up to Xebec.
"What is it, Captain?"
"Down there on the beach," Xebec instructed as he pointed with one hand while handing his spyglass to Whitebeard with the other. "Do you see that horned kid with the spiked metal club?"
"Yeah, I see him," Whitebeard answered as he looked through the spyglass. "What about him?"
"Go ask him if he wants to become a pirate. Then tell him I wanna see him."
"And if he refuses?" Whitebeard asked as he handed the spyglass back to his captain.
"Oh, he won't refuse. I got a really good feeling about him." Xebec lolled his tongue as he intensified his smile.
The battle on the beach went on for a time as Kaido fought that other pirate. That pirate felt the pressure and was gasping, so he decided to try and make nice. That was until Kaido booted him in the face, knocking him down. A large crowd had gathered to watch.
" . . . So strong . . . !"
" . . . Who is he . . . ?!"
"Hey, this says he's still fifteen!" A pirate exclaimed while studying Kaido's wanted poster.
"His bounty's that high for a fifteen-year-old?!" Another pirate exclaimed next to them. "This bounty is too high for a kid! He's a monster!"
"Hey, shitty brat!" Whitebeard called from nearby. "Do you wanna become a pirate?"
Kaido regards him.
"Rox said he wanted to see you," Whitebeard continued. "What do you wanna do?"
Kaido stared at him for a moment.
"Yeah, sure," he finally answered with a smirk. "Lead the way."
Whitebeard walked and Kaido followed. This stirred up the mood.
" . . . Shit! That's Edward Newgate of the Rox Pirates . . . !"
" . . . Rox wants to recruit that Kaido kid . . . !"
" . . . I wonder if that kid knows what he's gettin' into . . . !"
"So what's yer name, kid?" Whitebeard asked.
"Kaido."
"Kaido?"
"Yes, Kaido."
"Well, Kaido. My name is Edward, and I am the man who'll become the King of the Sea. Gura ha ha ha ha!"
They arrive at their destination. Nearby is a woman with a large saber hoisted on her shoulder.
"That's our ship, the Sally Rover," Whitebeard said to Kaido. "I know what yer thinkin' and it is a little banged up. Don't worry though, it can still float. Wait here a moment."
Whitebeard walked up the gangway and onto the ship.
"Hey brat!" the woman finally said to Kaido, getting his attention. "Do you have a mouth?"
She is a beautiful shapely young woman wearing a fedora with a polka dot pattern over a bandana. Long pink hair flows down, part of which is held in a ponytail as it flows over her coat she wears like a cape. Kaido stared at her, being somewhat stunned by the presence of a beautiful woman on this island since there were mostly men about the place.
"Hello?!" The woman called. "Can't you talk?"
"Ah, yeah, I can talk," Kaido finally answered snidely.
"How old are you?"
"Fifteen."
"Ha ma ma ma ma ma ma! Fifteen?! So young. Is this the first time teaming up with Rox?"
"Yes it is."
"He's a good-for-nothing. Don't trust him. If you have a problem, talk to me. Ma ma ma ma. My name is Linlin. I'm the woman who'll become the King of the Sea. Good to meet you."
"My name is Kaido. Good to meet you too."
"Hey, Kaido!" Whitebeard suddenly called from up on deck down the gangway then gestured. "Come on up!"
Kaido walked up the gangway, followed by Linlin. Once on board, there were more pirates around, five to be exact, with one of them hefting a silver axe and another a pretty woman as well.
"Hello there, rookie," that oily raspy baritone voice called, compelling Kaido to look off to one side.
He must be their captain. Kaido thought. He looks dangerous!
"Come on. Don't be shy," Xebec said while gesturing for Kaido to come closer with a broad tooth grin in a feeble attempt to sound jovial, but instead sounding as dangerous as he looks.
Kaido walks up to Xebec. The two are a match in height. A sweat drop trickled down the side of Kaido's cheek as he grimaced with discomfort, causing him to clutch his spiked club all the more tightly from the mental pressure that was building up in him from this Xebec fellow, who seems to be gazing through him rather than at him.
"So," Xebec began while standing arms akimbo, "your name is Kaido."
"Y-Yes," he answered.
"My name is Rox D Xebec, and I am the man who'll become the King of the World."
Kaido winced at that statement.
That Edward and Linlin both want to become the King of the Sea, but this one whats to become the King of the World? This one has his ambitions set much higher.
"I watched that little fight of yours from up here through my spyglass," Xebec continued as he pointed at Kaido. "I was gonna recruit him but I changed my mind and decided to recruit you instead, on account of you kicking his ass. I might have a place for you on my ship, Kaido, if you are able to pass my test."
"What test?" Kaido asked as uncertainty betrayed his expression.
"It's simple really." Then spreads his arms wide. "Hit me."
"Huh?!" Kaido winced with bewilderment.
"Put down your club," Xebec answered as he continued to spread his arms wide, "and punch me in the stomach as hard as you can."
Kaido frowned at Xebec then glanced around at the other pirates to see them standing there with poker expressions. Kaido faced Xebec and dropped his club. He slowly raised his arm with fist clenched as he pulled back.
He punched Xebec in the belly.
"Oy," Xebec sneered at Kaido as he lowered his arms slightly. "What was that? A love tap? You tryin' to flirt with me? You can do better than that. Now hit me harder." His tone firmer as he spread his arms wide once again.
Kaido again does so; this time putting more effort into his punch as he grits his teeth slightly.
"Oy, yoi, yoi," Xebec grumbled as his sneer grew more intense and he lowered his arms slightly. "Are you serious?! I invited you on my ship to test your potential and you give me love taps?!" His tone full of contempt. "Again!" Spreading his arms wide.
Kaido grits his teeth more intently as he winds up, muscles tense. With a loud grunt, he punched Xebec in the stomach.
That was when Kaido gasped at the suddenness of the blade of Xebec's sabre pressing against his neck. An ominous black aura wafts from Xebec, his face looking demonic.
"Maybe this will give you an incentive!" His tone matching his face. "You have one last chance to punch me in the stomach with all of the strength you can muster or your head will come off! And trust me, you won't move fast enough to pick up your club!"
Xebec moved his sabre away from Kaido's neck to spread his arms wide. Not needing to say anything more.
Kaido can feel the panic welling up in him and his thoughts race.
That first punch alone was enough to injure an ordinary man! I've defeated every man who ever challenged me! I've sent groups flying with a single blow! And now this Xebec didn't even flinch from my strongest punch! This man really can kill me! I . . . ! I . . . ! No! As he suddenly grimaced while raising his fist as his adrenaline kicked in. I will not die here! Because I will become the King of the Sea!
Kaido felt something awake within him, something flowing as his body suddenly felt even lighter from the strength that surged from within. With a scream, Kaido threw all of this newfound strength behind his fist, punching Xebec's belly. A gust of wind blew from that blow as his fist seem to sink slightly and movement could be felt from Xebec.
Then there was silence. Kaido lowered his fist expecting to die, seeing a slight fist print in Xebec's belly.
"Ra ha ha ha ha ha ha!" Xebec suddenly laughed with arms still wide. He lowered them while still grinning at Kaido. "Now THAT was the punch I wanted! Congratulations, Kaido! You passed! Ra ha ha ha ha ha ha!"
Kaido gaped with astonishment as his eyes are as wide as saucers with now shimmering pinprick irises.
That punch could've demolished a marine battleship and he took it like it was nothing! What the hell is he?!
Kaido looked back at the others, taking note of their poker expressions.
Odd, he thought. They should be happy that they have another crew mate.
"Now then, Kaido," Xebec said, getting his attention once more to see this monster make a small cut on his own forearm with his saber. "Hold out your left arm. I'm just gonna make a small cut like mine."
Kaido does so, if hesitantly, and Xebec does just that, then sheathes his saber.
"This is the blood oath," Xebec explained while showing his own cut. "By mingling our bloods, you will become an official member of my crew. Now let's do this."
Xebec and Kaido touch cuts. Kaido immediately gasped with a look of overwhelming shock and falls to the deck. He lies there for a moment looking stunned.
Wha-What the hell was that?! It felt like a million blades stabbing my soul!
Kaido slowly gets back up with a groan to face Xebec once again, who lolls his tongue.
"I was right about you the moment I laid eyes on you, Kaido," Xebec grinned broadly, dangerously. "You have immense potential that I can make use of. Like this."
Kaido immediately collapses to the deck once again and discovers that he cannot get back up.
"S-So weak!" Kaido gasped feebly. "Wha-What did you do to me?!"
"I just took all of your strength out and put it into me," Xebec explained in a mock casual manner while squatting down to face Kaido more closely. "But don't worry, I won't keep it all," as his tone softens, somewhat, while standing.
Kaido's expression is of shock as he suddenly recovers by being able to stand up once again.
"Is it a Devil Fruit power?!"
"Yes it is," Xebec verified. "I ate the Leech Leech Fruit. I'm a leech man. Anyone who mingles their blood with my own becomes bonded to me. Just like the rest of my crew. I can even return strength or pass it on to others. I have just distributed a portion of your strength to the rest of them, but will leave you with just enough to stand your ground. Whatever strength you gain from now on will be going into me. My Observation Haki is strong enough that I will know your actions beforehand, so if you ever think of doing something I don't like, I will be letting you know like I just demonstrated. My power is also like a dog's leash, so you won't be going far. You're my bitch, Kaido, just like the others. The only way out of this is through death but don't think you can commit suicide because you won't, or else the others could have just as easily done so to escape me."
Kaido gritted his teeth in frustration over this.
"But hey look on the bright side," Xebec said in a mock empathetic manner upon clasping both of Kaido's shoulders. "I chose you because you have what it takes to be a part of what has to be the most powerful pirate crew in the world! You will get to share in the spoils and the glory as we ultimately overthrow the World Government, making me the King of the World! Ra ha ha ha ha ha ha! Now let's weigh anchor!" As he released Kaido and turned to walk away. "We have a world to conquer! Ra ha ha ha ha ha ha!"
Notes:
I named Xebec's ship after a seventeenth century pirate crew called, the Sallee Rovers.
There is no doubt that Xebec had mastery of Armament, Observation, and Conqueror Haki but he would have needed much more in order to keep several dangerously powerful pirates, especially three future Emperors, under his control. So it makes sense to have a Devil Fruit that gives the eater the ability to leech off of the strength of others; especially since it took the combined, and direct, efforts of Garp and Gold Roger to defeat Xebec.
I have a theory that the lack of camaraderie amongst the Rox Pirates could have been from their inability to kill themselves, so they instead tried to kill each other.
Another theory I have is that when Xebec was finally killed, all of the strength that he had ever accumulated from his crew vanished with him, never returning to their original owners; meaning Kaido, Big Mom, and Whitebeard could have become even stronger if they had never joined Xebec's crew to begin with.