You're likely referring to the movie "One Piece 3D2Y: Red Letter Day"!
is a special television movie that fills in the gaps of Luffy's two-year training period. This guide covers where to watch the dubbed version, the story's significance, and its place in the series timeline. Where to Watch the Dubbed Version The English dub features the official Funimation/Crunchyroll One Piece 3d2y Dubbed
Where to watch: Crunchyroll (with dub settings) or Microsoft Store. You're likely referring to the movie "One Piece
This is a major point of confusion. The events of the One Piece 3d2y special—specifically the villain Byrnndi World and his crew—are anime-original (filler) . They are not mentioned in Eiichiro Oda’s original manga. Positive: Voice consistency with the canon Funimation dub;
The villain has a vendetta against the World Government and sees the weakened Straw Hat crew as easy targets. With his brother and a crew of powerful allies, World threatens to destroy the Straw Hats unless Luffy faces him. The special interweaves flashbacks of Ace, intense training sequences with Rayleigh, and a final battle that tests Luffy's new abilities.
(the "World Destroyer"), escapes from Impel Down level 6. His 100-fold power threatens the world and forces Luffy into a high-stakes battle while he is still mid-training.