In Wano, One Piece fan-favorite Roronoa Zoro unlocked an immense power up, ascending to new heights as a master swordsman. Engaged in battle with Kaido's strongest subordinate, Zoro gained awareness of his "kingly ambitions" and revealed his Conqueror's Haki.
As he was already strong enough, Zoro was immediately able to coat his attacks with the Color of the Conqueror, a feat that only the mightiest individuals can replicate. He went even beyond, creating a new sword style which involves him going all-out with Armament Haki and Conqueror's Haki simultaneously, reaching an even greater peak of strength.
Unleashing the true power of his Haki, Zoro lives up to the strongest One Piece characters
One Piece saw the rise of the King of Hell
During the battle between Worst Generation and Emperors, Zoro showcased attacks strong enough to seriously threaten Kaido's incolumity, as per Big Mom's own words. Even in an injured state after blocking the Emperors' combined move, Zoro was able injure Kaido, leaving him with a scar.
Still, owing to his Lunarian powers, which granted him a sort of invulnerability, King could endure all of Zoro's attacks, putting the latter into a serious predicament. Furthermore, one of Zoro's three swords, Enma, started draining his Haki on its own.
As such, Zoro remembered the words of Kozaburo Shimotsuki, a swordsmith who taught him that only weaklings who can't measure up to their own blades call them "cursed swords." This was the case with Enma, Kozaburo's greatest creation. The sword would test its wielder by draining his Haki, leaving him without energy should he lack the required power.
Realizing that Enma had been testing him, Zoro decided to unleash the real power of his Haki, satisfying the sword's will. Exerting a burst of Conqueror's Haki, Zoro caused the weak members of the Beasts Pirates to pass out instantly.
King asked Zoro about his "kingly ambitions," to which he answered that he must fulfill them to honor his promises to his captain and best friend. Coating his swords in Conqueror's Haki and Armament Haki simultaneously, Zoro unleashed the enormous power that laid dormant within him, employing the "King of Hell Style" for the first time ever.
Zoro created a new devastating mode based on Haki
Conqueror's Haki is the power to overwhelm. It can only be used by the few chosen ones born with the rare traits of a "Supreme King". The World's Strongest Swordsman is indeed among them, as the king of all swordsmen, who surpass all the others in might.
As hinted many times, from leaving Monet paralyzed in fear, to intimidating Queen, Zoro's dream to become the World's Strongest Swordsman makes him a true Conqueror. Kaido felt Conqueror's Haki from Zoro's Ashura attack, which involves the latter exerting his willpower to create the illusion of his body multiplicating like a demon.
Even among the "Supreme Kings," only a few are can coat their bodies, weapons and attacks with Conqueror's Haki, achieving a whole new level of power. The Color of Conqueror can only be trained to better focus it towards a specific target.
The overall power of one's Conqueror's Haki instead depends on the user's might, since it just grows stronger as the user becomes stronger. Thus, to use Advanced Conqueror's Haki a certain minimum standard of strength is needed. Impressively enough, Zoro could meet this requirement since the very start.
Being the future World's Strongest Swordsman and a natural-born Supreme King who can even use Advanced Conqueror's Haki, Zoro's potential is partly comparable to Luffy's, paralleling the dichotomy between "Dark King" Rayleigh and the Pirate King Gol D. Roger.
Kaido described the Advanced Conqueror's Haki as the signature ability of the absolute strongest fighters. Unlocking this rare power, Zoro gained a game-changing asset. With the King of Hell Style, he went even beyond, combining the Color of Conqueror and the Color of Armament.
The upgraded Haki makes Zoro a top tier fighter in One Piece
The new mode involves Zoro going all-out with his Haki. Zoro's Color of Armament was strong enough to block Kaido and Big Mom's combined attack, as well as significantly wound the former, even potentially threatening his incolumity.
With the further addition of the all-powerful Advanced Conqueror's Haki, Zoro's combat effectiveness skyrocketed. He now stands out as an absolute Haki master, which is awesome, given that Haki is the most deciding factor in One Piece fights.
While using this mode, Zoro's swords radiate the Conqueror's Haki typical black and red lightning spark trail, as well as a green smoke-like green flaming aura, which is caused by his Armament Haki.
Before his power up, Zoro used Ryuo unconsciously, as Enma exuded his Haki more than needed, enhancing his ouput. He is now able to do that consciously, even performing no-contact Haki slashes which struck the enemy without needing to physically touch them.
For instance, Zoro's normal Purgatory Onigiri features him dashing towards his foe and physically cutting him. The King of Hell Style Purgatory Onigiri instead becomes a projectile launched to slash at several meters away while Zoro remains completely stationary.
Zoro's new abilities also allow him to create Haki barriers, repelling his enemies without even touching them. The strongest attack he used with the new mode is the King of Hell Three Sword Serpent: Dragon Damnation, the upgraded version of his previous Three Sword Style: Ultra Tiger Hunt.
The Dragon Damnation involves Zoro jumping in the air and performing a descending slash, which unleashes Advanced Conqueror's Haki and Armament Haki simultaneously. Using this move, Zoro completely overpowered King's strongest attack, a giant dragon-shaped mass of Lunarian flames.
King's Lunarian flames are the same as magma, which in One Piece is stronger than normal fire, making the feat even more impressive. Even after having to overcome such an obstacle, Zoro's Dragon Damnation maintained enough power to slice through King's Lunarian-enhanced body and break his Armament Haki-empowered sword.
So far, these are the only King of Hell-enhanced moves that Zoro showcased. He also used an Advanced Conqueror's Haki-boosted version of the One Sword Style: Bird Dance, but that technique only involved the Color of Conqueror, without combining it with the Color of Armament.
One Piece fans are yet to witness the real power of Zoro's King of Hell Style, as the green-haired swordsman needed just a few hits to win the only battle where he used it. He annihilated King, a fighter of the same caliber as Marco and Katakuri, with just three strikes.
Given his clear superiority over Kaido's strongest subordinate, Zoro didn't even need to use the King of Hell Style versions of his strongest techniques, i.e., Death Lion Song, Billion-fold World Trichiliocosm, and Ashura, which he keeps preserved for more powerful foes.
Combining the Nine Sword Style: Ashura and the King of Hell Style, Zoro's power output reaches a Emperor-level peak. At this point in the series, he seems strong enough to put up a solid fight against most Admirals and Emperors, although he is yet to fully get to their level.
Either Admiral Fujitora or former Emperor Big Mom would be pushed into a very difficult fight, should they clash against the current incarnation of Zoro, who may very well be as strong as Oden. Moreover, Zoro has a lot of untapped potential to develop.
In the final saga, Zoro, as the future World's Strongest Swordsman and the right-hand man of future Pirate King Luffy, is set to surpass Mihawk (a man stronger than even Shanks), Rayleigh (the right-hand man of Gol D. Roger), and "Sword God" Ryuma (the legendary swordsman).
Summing up, why Zoro's King of Hell Style is a major thing in One Piece
Combining the advanced versions of Armament Haki and Conqueror's Haki in the "King of Hell Style", Zoro became a true master swordsman, entering the realm of the strongest fighters. He declared that he would literally "become the King of Hell” to keep his promise to Luffy.
This epithet has the same Japanese spelling of Silvers Rayleigh's "Dark King", which is similarly tied to the concept of the "King of the Underworld", an individual powerful enough to rule on his own, but who decided to be the "shadow captain" of a person he believes in.
This is exactly what Zoro did for Luffy, paralleling their counterparts in the older generation, Rayleigh and Roger. One Piece author Eiichiro Oda likely was inspired by the myths of Hades/Pluto with Zeus/Jupiter (depending on whether is ancient Greeks or ancient Romans) who are the King of Hell of their respective myths.
Things come full circle when Zoro's sword is called Enma, which is the Japanese name of Yama, the Buddhist version of the supreme ruler of the underworld, i.e., the King of Hell. This plays an umpteenth connection with Zoro's themes, as many of his techniques follow Buddhist terminology and imagery.