WARNING: The following contains spoilers for One Piece Episode 965, "Crossing Swords! Roger and Whitebeard!" now streaming on Crunchyroll and Funimation.
With the Whitebeard Crew busy helping care for Oden's two children, Episode 965 of One Piece, "Crossing Swords! Roger and Whitebeard!" actually takes a break from the titular characters to delve into some much-needed background for the arc's penultimate villain: the snake-in-the-grass usurper, Orochi Kurozumi.
In the episode, Shogun Sukiyaki, Oden's father, expresses his desire to have Oden succeed him as Shogun, but as Oden is out of the country, he appoints Orochi as the acting Shogun should his illness overtake him. This no doubt will surprise many viewers, as Sukiyaki is shown to be a rather level-headed man, yet he refers to Orochi as someone that Oden treated as "his little brother." Yasuie, a witness to this, is just as confused but accepts Sukiyaki's arrangement before asking Orochi why he never told Yasuie about his Kurozumi heritage.
Then, as One Piece is wont-to-do, the scene transitions into a flashback-within-a-flashback, showing Orochi's past. We follow him as he is chased down by Kozumi clan loyalists simply for being a Kurozumi, calling them the "vermin of the Land of Wano." Barely managing to escape, Orochi laments his life, wondering why he must suffer for his grandfather's crimes. It is then he spots an abandoned shack, and seeks shelter inside -- only to find it occupied.
Inside is an old woman and a silent old man with a biwa who scare the young usurper. The woman then reveals to Orochi his family's past -- or rather, the exact details of his grandfather's crimes. While the exposition is a touch clunky from its efforts to resist doing a flashback-within-a-flashback-within-a-flashback, the hag reveals that, prior to Sukiyaki's birth, there was no heir to the Kozuki clan, and as a result, the other daimyo were set to take the throne. Thus, the Kurozumi clan leader -- Orochi's grandfather -- began killing off the other potential pseudo-heirs, so that when the time came, he and he alone could inherit the throne.
But after successfully poisoning the rest of the daimyo and making it look like an inside job -- who should be born but Sukiyaki? With the heir now secured, when Orochi's grandfather's treachery was discovered, he committed seppuku while the rest of his family was hunted down.
In telling Orochi this story, the woman warps the direction of his anger away from his grandfather, who was clearly just trying to take his rightful place in power, and onto Sukiyaki, the one whose birth prevented power from being given to his family, and by extension, to him. This lure of "what could have been," coupled with the woman's promise of power using her ability to mimic any face and the oh-so-convenient Snake-Snake Fruit plopped at his feet, Orochi finally decides to stop wallowing in pity and running away, opting instead to use his acting skills to work his way up the ranks of society, while gathering money to create a weapons plant, as per the old woman's plan.
And so it is that, after stealing Yasuie's funds and preying upon Oden's generosity, that the woman transforms herself into Oden to visit Sukiyaki in the Capital, introducing Orochi as someone who is like a "little brother" to him -- but that is by no means the end of the story. It's unknown how many times we see Sukiyaki alive after Orochi's entry into the palace, as a later scene reveals that the woman has copied Sukiyaki's form -- and is faking his illness. For all we know, Sukiyaki was poisoned mere hours after Orochi's entrance into the Flower Capital. It also explains why the woman-disguised-as-Sukiyaki's coughs so fake sound to the casual observer prior to this reveal. As a clone arranging for Orochi's ascendance to power, she clearly wasn't as good of a performer as Orochi himself.
And so it is that we finally understand how all the pieces fit in place that an absolute nobody like Orochi -- sniveling, conniving Orochi -- could seize power so easily from the legendary Kozuki Clan, and with it, seal Wano's blood-soaked future.