Later that evening, Jack gathered everyone inside the new training room he had built earlier.
The room was spacious, its walls reinforced with 2 layers of different metal, the first one is Uru Metal and the second one is Vibranium.
"This," Jack explained, pointing at the dial, "is a gravity regulator. I can increase the gravity up to ten times normal gravity. It's perfect for training speed, strength, and endurance."
Zoro face lit up like a kid at a sword shop. "Finally, something worth my time."
Sanji raised an eyebrow. "You made this? On a ship?"
Jack nodded casually. "And it's fully powered by the arc reactor I installed. No matter how much power it uses, we'll never run out."
Luffy's eyes sparkled. "COOL! Can we try it now?!"
Jack smirked. "Sure. Gravity x2."
He twisted the dial, and everyone felt the sudden heaviness push them down. Luffy immediately started laughing and bouncing around, completely unaffected.
"WOOO! It's like a fun game!"
Zoro grunted and dropped into a push-up position. "Perfect for weight training."
Sanji took a step and nearly stumbled. "Tch. Not bad. Good for leg strength."
"Arghhh... I can't do this," said Usop. "Nami let's go out."
Jack stood calmly, unaffected. The golden tattoos on his arm flared faintly. "Let's see how well my Cosmic Strength works under pressure. Zoro, Sanji, spar with me."
"Oi, you sure about that?" Usopp said nervously, stepping back. "If you break the room, we'll sink!"
Jack smirked. "I reinforced the walls. Trust me."
Zoro drew two of his swords, grinning. "Don't hold back."
Sanji cracked his knuckles. "I'll aim for your head."
They attacked at the same time, Zoro from the left, Sanji from above.
Jack moved fast.... faster than they expected. His golden arm glowed, blocking Zoro's slash with a metallic clang, then countering Sanji's kick by grabbing his leg mid-air and flipping him backward.
Sanji landed gracefully, scowling. "You're faster than you look."
Zoro grin widened. "Good. I like a challenge."
He switched to Three-Sword Style and charged. "ONIGIRI!"
Jack golden fist met Zoro's blades head-on, the impact caused a shockwave luckily the room are ok.
Everyone outside the sparring circle gasped.
Luffy was practically shaking with excitement. "AMAZING! DO IT AGAIN!"
Sanji joined the attack, his legs blazing with speed as he unleashed a barrage of kicks. Jack blocked most of them, but one connected with his ribs — the force actually pushed Jack back a step.
Jack smirked. "Not bad."
Then his golden gauntlet flared fully, and he threw a punch, this time aiming at the floor.
BOOM!
The entire training room trembled, the room was unaffected.
Zoro and Sanji froze for a second, shocked.
"…Okay," Zoro admitted, sheathing his swords, "This is so good"
Sanji clicked his tongue. "Yeah."
At this time Sanji and Zoro are still good crewmate, they didn't have rivalry among them yet. Give them few days, maybe they will start bickering each other.
"Jack! Now fight me!" said Luffy.
Jack shook his head, his golden tattoos fading away. "No, not yet."
Luffy pouted. "Why not?!"
Jack's expression turned serious. "Because if you want to fight me properly or anyone strong in the future, you'll need something first. Haki."
Everyone froze.
"Haki?" Zoro asked, frowning.
"What's that?" Sanji intrigued.
Jack didn't answer right away. Instead, he waved his hand. Golden light pulsed around his fingers and suddenly the room darkened.
Once again he activated his illusion
They were standing on a battle between Zoro and Mihawk.
"This," Jack said, "is observation haki."
They saw Zoro charging, his eyes locked on Mihawk sword. Zoro parried and dodged using his eyes and reflexes. Mihawk, however, barely moved, stepping aside with effortless precision, as if he already knew exactly where Zoro swords would strike.
Jack pointed. "See? Zoro relies on what he sees. Mihawk relies on what he feels. Observation haki lets you sense if someone wanted to attack you, to know an attack is coming before it even moves."
The crew watched in silence, then the illusion shifted... Zoro three swords met Mihawk dagger.
"Now this," Jack said, "is armament haki. Mihawk's dagger isn't just a piece of steel. It's coated in invisible armor. That's why Zoro's blades couldn't overpower it."
He turned back to the crew. "Armament haki can harden your body or your weapon making it strong enough to block even the sharpest sword or punch through even the toughest skin. It's invisible… but it's what separates the strong from the weak."
"Mihawk got a title of the strongest swordsman by mastering only two haki but there is someone who mastered 3 haki," said Jacl
He raised his hand again, and this time a new illusion appeared, a scene none of them had never seen before.
Monkey D. Garp, standing on a battlefield, his fists clenched.
Luffy's face went pale. "G-Gramps…"
"Watch," Jack said.
Garp leapt high into the air and slammed down a single punch, a Galaxy Impact. Half the island below him shattered, leaving a crater and shockwave that sent hundreds of pirates flying.
Then the scene shifted, Garp blitzing through a giant enemies, each blow sending massive bodies flying as if they weighed nothing.
"This," Jack said, his voice low, "is the power of someone who mastered armament and observation haki and the most powerful haki which is conqueror.
The illusion faded, leaving the room silent... Jack turned to Luffy.
"If you want to become Pirate King, you must learn haki. Otherwise, the Grand Line will eat you alive."
Luffy's grin had faded. He was actually sweating now, but he clenched his fists.
"Then I'll learn it. I will."
Jack then turned to Zoro. "And you. If you want to defeat Mihawk one day, you can't rely only on your swords. You'll need haki to cut what can't be cut."
Zoro's hand gripped Wado Ichimonji tightly. "Tch. I get it."
Sanji exhaled smoke and nodded. "I want to learn too."
Even Usopp, though trembling, raised his hand nervously. "D-Do I have to learn too?"
"Yes," Jack said simply. "You all do. Haki isn't just power... it's survival."
Luffy's grin returned.
"Alright then! Starting tomorrow, we train until we master haki!"
"What is conqueror haki?" asked Zoro.
"Conqueror haki is a haki that 1 in a million have it," said Jack. He then showed Garp and Gol D Roger, clashing the conqueror haki and even wood splintered.
Everyone looked with amazement.
"How do I awaken the conqueror?" asked Zoro.
"That is I don't know but those who had awaken it had similar trait, either they wanted freedom, wanted to become conqueror, wanted to become very strong and you Zoro and also luffy had that same trait," said Jack.
"You mean I could awaken it?" asked Zoro.
Jack nodded, "If you manage to awaken conqueror the possibility to win the fight against mihawk is higher because he is someone who cannot use conqueror haki."
"What, he didn't have conqueror eventhough he is that strong?" asked Sanji.
"Yes... his mastery is on armament that even if he meet someone who had conqueror... he didn't lose witht them.." replied Jack.
"Okay, let's train," Luffy said.
"Where is Nami?" asked Jack.
Nami had left them and didn't want to join the training.