The Straw Hat Pirates have been having a tough time in the One Piece manga as the Whole Cake Island arc enters its climax, and Luffy has been dealing the the worst of this as he's fighting Katakuri in the mirror world.
As the fight progresses, Luffy has been gaining a new Haki power, and right before he reaches that ability again he flashes back to Silvers Rayleigh, his teacher that fans haven't seen since the time jump.
Chapter 894 flashes back to a moment Rayleigh was training Luffy. Luffy is starving, but Rayleigh says that Luffy can't eat until he's mastered sensing attacks with his Haki while blindfolded. Luffy is hilariously inept, as the focus on food leads him to punching Rayleigh's food and messing up the place.
Afterward it's revealed that Luffy is able to feel the emotions of living things, and that mastery of the observation haki would allow him to perceive attacks and even predict the future. But he worries what Luffy would do if he were to come across such an opponent with only two years of Haki training.
Luffy's answer to this is to just see what kind of guy his enemy is, and Rayleigh laughs that he didn't even think of that. Just as the flashback ends, Luffy successfully dodges a rain of Katakuri's punches and begins his counterattack in full.
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has been collected into 87 volumes, with a few chapters yet to be included. It 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 430 million copies sold worldwide.
Eiichiro Oda also revealed some upcoming news for One Piece's future at Jump Festa 2018. Not only will the manga see Wano sooner rather than later, the next villain of the series is "one of the legends which lurks in the One Piece world. The greatest enemy ever for Straw Hats will hinder their way...Can you believe Marineford Summit War will look 'cute' compared to that?"
One Piece is currently in the middle of the "Whole Cake Island" arc and is now streaming on Crunchyroll and FunimationNOW.