Luffy uses Conqueror’s Haki only during the Marineford Arc, but it has a massive impact. It occurs in Chapter 569 (Episode 478) .
Both executioners fainted mid-swing, buying Luffy time to reach the platform. when does luffy use conqueror's haki in marineford
Luffy has just lost his brother, Portgas D. Ace, to Admiral Akainu’s magma fist. Overwhelmed by grief, trauma, and rage, Luffy collapses into a catatonic state. As Ace’s body lies nearby, Luffy’s willpower—pushed past its breaking point—unleashes a wave of Conqueror’s Haki unintentionally. Luffy uses Conqueror’s Haki only during the Marineford
As Luffy rushes toward the execution platform where Ace is bound, he is intercepted by the Executioners and a massive mob of Marines. Just as Admiral Kizaru is about to intervene, Luffy stops. Luffy has just lost his brother, Portgas D
It explains how Luffy was able to later (in the Fish-Man Island Arc) knock out 50,000 opponents at will . Marineford was the seed—unconscious and raw—that would eventually bloom into mastery during the timeskip.
The impact of this unrefined Haki was so immense that it instantly changed the tide of the battle:
The trigger for this awakening is pure desperation. As Portgas D. Ace is about to be executed on the platform, Whitebeard—already heavily wounded and suffering from a heart attack—is unable to use his own Haki to intervene. With no one else close enough to stop the blades, Luffy screams, .