Haki/Haoshoku Haki

Haoshoku Haki is a rare form of Haki that cannot be attained through training and only one in a million people carry it. It was first demonstrated by Shanks, when he used it to scare off a Sea King. Later on, during a meeting with Whitebeard, he used it to knock out most of the crew, which the few still standing attributed to his superior Haki. Luffy first displayed the effects of this type of Haki against Duval's bull, Motobaro, much to everyone's confusion. He displayed it again on Amazon Lily while battling the Boa sisters, knocking out a large portion of the audience observing the match. This is also the first time it is referred to by name. Luffy's exhibition of it during the Marineford War was enough for the Marine admirals to become wary enough of him that they prioritized his elimination afterwards.

In the Viz Manga, this is called Color of the Supreme King. In the FUNimation subs, this is called Conqueror's Haki.

After the timeskip, Luffy is proficient enough to tame monsters, as well as knock out a selected group of individuals without affecting any of the bystanders using Haoshoku Haki, displaying his new-found proficiency in the skill.

This type of Haki grants the user the ability to dominate the wills of others. The most common usage in the series so far is knocking those with weak wills unconscious. Whilst inexperienced users are restricted to merely overpowering the will of one individual or blindly knocking out weak-willed people around them, those with more expertise can pick out weak-willed individuals in a large group and knock them out without affecting those around them. However, those with stronger wills can resist or even ignore the effects of Haoshoku Haki. The ability to knock someone out depends on the difference in strength between the person using Haki and the person or persons the user is trying to knock out. The greater the power gap, the easier it is to knock the victim out. According to Rayleigh, while this type of Haki cannot be attained through training, it can be improved through training, as it grows as the user's spirit grows. It seems that upon recovery, victims of Haoshoku Haki experience the sensation of chills running through their body. However, it presumably wears off after a short period.

Known Haoshoku Haki Users
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Anime and Manga Differences

 * In the manga, those hit with Haoshoku Haki form foam at the mouth after or while losing consciousness, whereas in the anime, they simply faint.
 * In the anime, use of Haoshoku and Busoshoku Haki creates a rippling or shockwave-like effect, similar to the Gura Gura no Mi, while in the manga, it is completely invisible. Also, in recent episodes, use of Haoshoku Haki causes the affected area, excluding the user, to gain a dark blue tint. However, this could just be for dramatic effect. In the One Piece Film: Z, the Haoshoku Haki's use creates a circular purple wave.

Translation and Dub Issues
Though the "Haō" in "Haōshoku" is translatable to "king", it refers to a "warring king" or "conqueror", one who rules by power and conquest. The term "Overlord" has been used by some translators.

Trivia

 * Curiously, all the canon Haoshoku Haki users aside from Luffy and Chinjao share a royal position or epithet, which indicate their dominating role:
 * Boa Hancock, known as the Pirate Empress, is Amazon Lily's ruler as empress and the captain of Kuja Pirates.
 * Donquixote Doflamingo is the King of Dressrosa and the captain of Donquixote Pirates.
 * Edward Newgate was one of the Yonko and the captain of Whitebeard Pirates.
 * Portgas D. Ace, being the son of the Pirate King, can metaphorically be considered a prince. He was also the captain of Spade Pirates.
 * Shanks is one of the Yonko and the captain of Red Hair Pirates.
 * Silvers Rayleigh has the epithet of Dark "King".