WARNING: The following contains spoilers for One Piece Chapter #1010 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
One Piece Chapter #1009 was a fast-paced team assault by the Supernovas in an effort to knock out the Yonko, Big Mom. Though an impressive display of both teamwork and strategy, in the process, Zoro suffered some alarming damage while blocking an attack from both she and Kaido. Now, in Chapter #1010, the pain is affecting him as he and Traffy are left alone to fight Kaido. But that doesn't prevent the samurai, or his captain, from surpassing their limits. Both Luffy and Zoro advance their haki somewhat, and it's well earned. Having gone through some intense training during Wano, these more well-honed techniques can serve our captain and first mate well going forward.
The set-up for this is Big Mom's continuing fall to the ocean below her. Prometheus tries to save her in the air but is prevented by Zoro's continuous cutting of the fire homie. Law is surprised that Zoro can even move given how powerful the combined attack by the two Yonko was. Kaido steps in to save Prometheus, but Law's Op-Op Fruit powers let him teleport Zoro out of harm's way, while also striking Kaido's neck.
Unfortunately, Prometheus gets away but Law is fine with this: all they needed was to separate Kaido and Big Mom to make them more manageable to fight. With that achieved, Kid and Killer chase after Prometheus, leaving Law and Zoro to face Kaido and protect an unconscious Luffy.
His captain's life threatened by Kaido, Zoro steps up to face the dragon human hybrid. With all three swords in hand, the swordsman releases a technique we haven't seen Zoro use since before the timeskip: Demon Aura Nine Sword Style. Manifesting two additional Zoros -- with three swords each -- was how he defeated the CP9 agent Kaku at Enies Lobby, and it's by far his strongest attack.
When Zoro unleashes the simultaneous nine sword swing, known as Dead Man's Game, on Kaido, the Pirate Emperor receives a deep flesh wound to his chest. Even though he remains on his feet, Zoro actually leaving a mark is still impressive. Kozuki Oden is the only other person to have done so.
Kaido is so flabbergasted, he postulates that Zoro used Conqueror's Haki or The Color of the Supreme King. Zoro doesn't know what Kaido is talking about, but it's quite possible that Zoro's ultimate technique could really be a manifestation of his Conqueror's Haki -- if he does actually possess it.
Unfortunately, all of Zoro's stamina is completely spent with that technique, so he can do nothing to defend himself from Kaido's next move. Law steps in to try and save the swordsman, but Kaido's ready for the surgeon this time, striking him down with Thunder Bagua.
As the two Supernovas lie face down on the ground, unable to move, Kaido only sees them as a waste of potential for going against him rather than allying themselves with him. But the chapter has one more surprise up its sleeve: Luffy gets back to his feet, promising the tyrant Yonko he'll be the one losing this fight.
As Luffy steps up to protect his friends, he realizes the secret behind Kaido's swings is that he can infuse his weapon with his Supreme King Haki. In this moment of desperation, and remembering the words of old man Hyou, Luffy's own haki evolves, and with every punch, he can damage Kaido without even touching the dragon-human hybrid's body. Luffy confidently assures both Zoro and Law that he can handle Kaido on his own now.
It's still not completely certain at this point whether Zoro's Demon Aura is a manifestation of Supreme King Haki, but if it is, it could be a game-changer for the Straw Hat crew. Supreme King Haki is very rare, but Roger's crew had three known members who could utilize it -- Roger, Rayleigh and Shanks. Who could be more suited to joining their ranks than the world's greatest swordsman and first mate to the future King of the Pirates?