One Piece's anime has gone back to the Wano Country after a brief detour promoting the release of One Piece: Stampede in Japan, and Luffy has gotten into the spirit of the samurai influenced region in full. In the attempt to cure Otama's illness, Luffy has to cross the wastelands of Wano in order to get her into town and see a doctor. To keep from sticking out like a sore thumb, Otama's master Hitetsu helped Luffy into appropriate samurai garb.
But with this samurai look, Luffy wanted a sword to go with it and takes a powerful and cursed sword, Kitetsu II (otherwise known as the Nidai Kitetsu), into his possession and doesn't really care about its cursed and dangerous past.
In Episode 897 of the series, Luffy's Haki sense has him find the Kitetsu II hanging on Hitetsu's wall. While he takes it nonchalantly and prepares to head out, Hitetsu warns him that it's a "sin" to handle such a well known sword so casually. He notes that his ancestor, Kotetsu, forged the sword and it's actually one of the 21 Excellent Grade Swords.
The Kitetsu II is one of the few swords made by "the great swordsmiths." Known as the Grade Swords, these blades are separated into separate levels of rareness. There are the 50 Fine Grade Swords, the 21 Excellent Grade Swords, and the 12 Supreme Grade Swords. Hitetsu tries to explain the important story behind Kitetsu II in particular, but Luffy hilariously runs away before he can finish the story.
Hitetsu chases after Luffy in order to get him to drop the sword, noting that it's cursed and he seems quite scared about what this means. But as usual, Luffy pays these kinds of warnings no mind and hilariously tosses Hitetsu to the side with ease and runs off with the sword. It's clear that he won't be returning the sword, and Luffy thus hilariously "inherits" the sword in a moment of pure piracy.
Luffy will soon get his chance to put his samurai skills to the test as not only as he reunited with Zoro after spending more than two years apart in the anime, but Hawkins of the Worst Generation has arrived to fight the two of them in the Wano wasteland. As 897 comes to an end, Luffy's ready to go all out in samurai action with this new sword.
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has since been collected into over 80 volumes, and has been a critical and commercial success worldwide with many of the volumes breaking printing records in Japan. The manga has even set a Guinness World Record for the most copies published for the same comic book by a single author, and is the best-selling manga series worldwide with over 430 million copies sold. The series still ranked number one in manga sales in 2018, which surprised fans of major new entries.