[S3E12] The Conqueror
Kang was the supreme Leader of Earth in the 31st Century where he has conquered multiple worlds and fought a thousand wars. In the past, he has desecrated the Avengers' image by making them out to be rogue outlaws, by importing false facts in the Avengers museum. However, Thor, the last original Avenger alive, revealed the truth about the lies Kang made up about them and led a group of rebels against the conqueror.
[S3E12] The Conqueror
Kang in particular, sent Falcon a few decades into the future to a mostly deserted New York that he ruled using tech dampeners to block any technology aside from his. There he needed his help in saving the world from a rip in the universe caused by Kang's constant time travel. Falcon reluctantly agreeed to join him in exchange of letting the people of New York free, return to his time, and never time travel again. For the next few years, Kang trained him in different forms of combat as he defended Kang from others such as Red Hulk who did not trust the conqueror's way doing things.
In an effort to wipe the Stark lineage from history, Kang sent robots disguised as Hydra agents to the past to kill Howard Stark thus erasing the Arc Reactor, Iron Man, the Avengers and even his old adversary Arno Stark from ever existing. However one of his robots became damage and accidentally sent Howard and Agent Carter to the present day. Kang attempted to kill Howard (or at least his son Tony) himself but was outmatched by the combine efforts of both Starks in Iron Man Armors, Agent Carter and Captain America. Kang settled by having Tony be erased by the paradox of having Howard displaced in time for too long and escapes through a time portal, Kang returned for one last chance to kill Howard before he and Agent Carter were sent Back, but was stopped by Arno Stark who dragged him back into the future as the conqueror cursed all the Starks.
Now we all know that in truth, Oda had not yet defined Haki by then, but the canon position is that Zoro used armament haki vs Mr. 1. This is poetically mirrored again here with CoC. It makes sense that luffy figures out how to coat himself with CoC to fight Kaido, because Zoro showed him how on the rooftop. When Zoro says 'What do you mean' to Kaido, he doesn't say it as, 'What do you mean conqueror's haki? I don't have it'. It was more that just like with Mr 1, and just like us the readers, Zoro did not know that you could use conqueror's haki as a tool- He did not INTEND to use conqueror's as a tool against Kaido when he did asura- he just wanted to hit him with the best he had. That is why he is surprised that Kaido says he has CoC just from seeing him do asura, because he didn't think he was doing it.
The whole fight, the gang put up a real fight, but even ryuo was shown as not enough to sufficiently hurt-hurt kaido, even though it does affect him. Zoro using CoC with Asura showed Luffy this because he says 'he finally gets what happened'. basically, Luffy puts 1+1 as 'Kaido said Zoro used conqueror's haki with Asura, which means you can put CoC on a sword, which means you can put CoC on a kanabo... OH SHIT NO WONDER MY ARMAMENT HAKI COULD DO JACK SHIT AGAINST THUNDER BAGUA ... OH FUCKING SHIT THAT MEANS YOU CAN PUT COC ON A FIST (pause).' 041b061a72