WARNING: The following article contains spoilers for One Piece, Episode 910: "A Legendary Samurai! The Man Who Roger Admired!"
The last episode of One Piece was a trip! In it, we learned the secret history of the Land of Wano and how, years prior, the old rulers and samurai were slain by the current shogun, Orochi. With the old castle burned and ruined, it would seem that nothing would remain of the old rulers of Wano.
Except for the fact that the samurai Orochi "slew" were destined to return 20-years after the fact. It turns out that Kin'emon and his crew -- along with Ochiku -- are in reality the very samurai destined to avenge their former masters. Oh, and one more important thing: it turns out that the samurai traveled two decades into the future to this very moment to exact their revenge.
THE HISTORY OF KURI
We start One Piece's latest episode at Oden Castle, where the Straw Hats, now united, talk strategy. Everyone seems stunned by the time travel revelation, demanding to know how this could have happened. This leads to Kin'emon telling them about the burning of Oden Castle -- originally called Kuri Castle, but nicknamed after Kozuki Oden, the son of the Shogun. However, due to Oden's habit of starting fights, he was eventually banished, which he actually took very well. He went to Kuri, the lawless area of Wano where criminals ran rampant and ended up fighting the criminals, establishing order to the area.
Oden challenged Ashura Doji, the most dangerous man in Kuri, to a fight and, unsurprisingly, won. Doing so also won him the respect of everyone else; putting all of Kuri under his loyal command. Once in charge, he trained the ruffians to work and till the land, developing Kuri and Paradise Farm for all. Outsiders felt welcomed in his land under his caring rule. Oden became the Daimyo of Kuri at only 20-years old, gaining the respect of even Gol D. Roger and Whitebeard.
Still, Oden was restless in his new position; wanting to explore.
THE EDEN OF ODEN
This is when Kaido and Orochi attacked, setting up Oden in a sneak attack. They burned Oden's castle down and rebranded him as a criminal. Orochi executed Oden before Wano's populace -- all 20-years prior to now. But Oden, rather than accept death, met it in combat head-on. Those most loyal to him ran straight to Oden Castle, only to find it on fire.
The last guard attacked Kaido's forces, fighting to salvage the castle, but they could only save Oden's son, Momonosuke, from the fire. Oden's wife, Toki, who possessed the power of the Time-Time Fruit, sent Oden's loyalists into the future. She, on the other hand, remained behind to burn so she could be reunited with her husband in the afterlife.
In the future, these loyalists arrived amid the ruins of their old castle and witnessed how their old Wano had been destroyed in the years that had passed. From there, they plotted their revenge, gathering soldiers to the cause of overthrowing Wano and Orochi -- soldiers like the Straw Hats.