Monkey D. Luffy first learns about Haki and begins his training during the time skip in the One Piece series. This training takes place on the island of Rusukaina under the guidance of Silvers Rayleigh. The time skip occurs between episodes 516 and 517 in the anime. Specifically:
- In episode 516, Rayleigh introduces Luffy to the concept of Haki and demonstrates the three types: Observation Haki, Armament Haki, and Conqueror's Haki.
- After this, during the two-year time skip, Luffy trains rigorously to master these Haki abilities.
Before the time skip, Luffy had unknowingly used Conqueror's Haki on several occasions, but it was during and after the training that he became consciously aware and skilled in its use. Later, Luffy also learns advanced Haki applications in the Wano arc, such as emitting Armament Haki to attack opponents from a distance and using Inner Destruction. So, the key episode where Luffy officially learns Haki from Rayleigh is episode 516. He then masters it during the time skip and uses it extensively in later arcs. If you want the first time he uses Armament Haki, that happens a bit later, around episode 525. This timeline captures the beginning of Luffy's Haki journey clearly.
