Warning: Contains spoilers for One Piece chapter #1042
In One Piece, Kaido is recognized as the strongest pirate in the world, but his dream to have a fair fight against a worthy opponent has just been shattered by the actions of the World Government. Even worse, something similar happened in the past, and Kaido is now reliving his worst memory.
Kaido is the Governor-General of the Beasts Pirates, a massive fleet that includes many Devil Fruit users. Because of his military power, he is one of the Four Emperors who rule over pirates, but his personal strength is what gained Kaido his title "the strongest creature". He has the power of the Uo Uo no Mi, Model: Seiryu, which allows Kaido to transform into a dragon, and he can also use Haoshoku Haki, an ability that only one in a million possesses. His goal is to amass enough military power to topple the order of the world, and to achieve that he took over the island of Wano as his base to transform it into a weapons factory and to make it a "pirate paradise". However, in the clash with Wano's defenders and the legendary samurai Kozuki Oden, Kaido suffered humiliation, a stain that he was never able to erase.
Oden was the rightful daimyo of Wano, but he was denied that position by the machinations of his enemy Orochi, who allied with Kaido. When Oden learned of Kaido's true plans, he moved to confront him along with his faithful retinue of samurai. In the battle that followed, Oden wounded Kaido, something that only five people in the world were ever able to achieve. However, Oden was then distracted by a trick pulled by his enemies, and Kaido blindsided him, ending the fight. Despite this victory, the memory of Oden remained with Kaido for decades, to the point that he can perceive his presence emanating from his sword, Enma, now wielded by Zoro. Kaido is a ruthless man, but he believes in strength above anything else. The fact that he was unable to settle the fight with Oden fair and square keeps nagging at him, especially because he was not able to find another worthy opponent since.
This changed when Luffy showed up at Kaido's base, boldly declaring that he would take down not just Kaido, but also his fellow Emperor, Big Mom. Kaido has beaten the Straw Hat Captain in the past, and he still seemed superior, but Luffy is finally unleashing his true power, having mastered Haoshoku Haki and gained more control over his Devil Fruit. Kaido is genuinely enjoying the fight, feeling truly challenged after so many years, but in chapter #1042 he sees his dream shattered when an agent of CP0, a top-secret World Government organization, interferes and distracts Luffy. Kaido sees it, but he cannot stop his last attack, which takes Luffy down. This is exactly what happened with Oden, and Kaido's expression when he realizes what just happened is one of pure anguish. He saw Luffy as the chance to get rid of Oden's ghost once and for all, and now his worst memory just repeated itself in front of his eyes.
The final double page of the chapter is one of the best in the history of One Piece. Kaido's memories juxtaposed with the present, as Luffy's face looks exactly like Oden's. Kaido is often torn between the honor that comes from being the strongest and his ruthless nature as a pirate who will do anything to achieve his goals. If Luffy recovers from that last blow, Kaido's anger at the CP0 agent could be the distraction he needs to end what is without a doubt the most important fight in the history of One Piece.