Haoshoku Haki

Haoshoku Haki is a rare form of Haki that cannot be attained through training. Only one in several million people have this ability. This type of Haki allows the user to exert their willpower over others.

It is said that whoever possesses this type of Haki has the qualities of a king.

Background
It was first demonstrated by Shanks, when he used it to scare off a Sea King, though it was not fully explained and revealed. 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 Sandersonia and Marigold, 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.

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.

Usage
This type of Haki grants the user the ability to dominate the wills of others. The most common usage of it shown in the series so far is using it to exert the user's willpower onto those with weak wills and rendering them 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 the others.

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 the 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 upon through strengthening the will of the user. 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. One can also use Haoshoku Haki to destroy things as seen by Shanks when he was able to crack a part of Whitebeard's ship using his Haki.

Strengths and Drawbacks
One of the main advantages of this form of Haki is that it can be used to knock weak willed people out without physically harming them such as when Luffy used it to knock out the Fishmen Island civilians who mistakenly believed he had kidnapped Princess Shirahoshi. It also allows the user to avoid having to fight with people weaker than themselves avoiding unnecessary conflict by ending a fight before it starts, such as when Luffy used it to knock out Demaro Black and the Impostor Straw Hat Pirates. As shown with Luffy's use of it on the New Fishman Pirates, this form of Haki can also be used to reduce an enemy's manpower.

However these advantages only apply to weak willed people, as shown by Luffy's use during the Whitebeard War, while Luffy was able to knock out weak willed Marines like Jango, the Marines with stronger wills like Fullbody were able to remain conscious.

Another advantage is a proficient user like Luffy can use it to tame powerful and dangerous animals, allowing the user to pacify ferocious beasts such as the Kraken or Fighting Bull while turning them into powerful allies.

Other Information
There have been two ways in which this Haki has been used. The first and most common way is in a quick burst that will knock out those with weaker wills relatively easily. The second method is to release it continuously, which will not only knock out those of weak wills around the user, but also put pressure on the surroundings, actually affecting other physical objects besides living beings. It also has been shown that light bursts of Haoshoku Haki can be used in order to tame or intimidate creatures, and exhibit the users' superiority over the beast(s).

It has also been shown that when two users of this Haki clash, it causes a rippling or shock wave effect cracking the air similar to the Gura Gura no Mi. The shock waves from the Haoshoku Haki clash between Luffy and Chinjao were strong enough to blast people out of the coliseum arena into the water below. The Haoshoku Haki clash between Luffy and Doflamingo produced shockwaves that pushed Trebol and Law's incapacitated body away from them.

According to Trebol, Haoshoku Haki is proof of an individual who possesses the qualities of a king, a candidate chosen by heaven. Indeed, most of the people who have been shown to possess Haoshoku Haki (aside from the non-canon Naguri) share a royal position or epithet or attained some highly respected status, which indicate their dominating role: While this is shown to be a very rare gift among the world as a whole, it is noted by Don Chinjao that the New World is filled with numerous others who possess the ability to use this Haki. He explains that Luffy will most certainly encounter them and fight them for the title of Pirate King.
 * Boa Hancock, also known as the Pirate Empress, is Amazon Lily's ruler, a member of the Oka Shichibukaiand captain of the Kuja Pirates.
 * Charlotte Linlin is Totto Land's queen, one of the Yonko, and captain of the Big Mom Pirates. She is also the matriarch of the Charlotte Family.
 * Charlotte Katakuri is one of the Sweet Commanders of the Big Mom Pirates (noted to be the strongest one), the minister of Komugi Island and the second son of the queen of Totto Land.
 * Chinjao was a Don before passing the title to Sai. He is the leader of the Chinjao family and former 12th Admiral of the Happo Navy.
 * Donquixote Doflamingo is the former king of Dressrosa, a former member of the Oka Shichibukai and captain of the Donquixote Pirates, as well as a former World Noble and the most influential person in the Underworld.
 * Edward Newgate was one of the Yonko and captain of the Whitebeard Pirates. He was the only man strong enough to rival the Pirate King Gol D. Roger, and was also known as the one closest to the One Piecebefore his demise.
 * Monkey D. Luffy is the captain of the Straw Hat Pirates and leader of the Straw Hat Grand Fleet, as well as the sworn brothers of Sabo and the late Portgas D. Ace, the grandson of Vice Admiral Monkey D. Garp who has the epithet "Hero of the Marines", and the son of the Revolutionary Army leader, Monkey D. Dragon. Also, he has made notable friends with many royal people, such as Nefeltari Vivi, Shirahoshi, and Rebecca, as well of having former royals of his crew and fleet, Vinsmoke Sanji and Cavendish, respectively, and is the current possessor of the straw hat that once belonged to the late Pirate King, Gold Roger, which was later given to Shanks and later given to Luffy. He has also been recognized as the Fifth Emperor (Yonko) of the Sea.
 * Portgas D. Ace, being the son of the Pirate King, can metaphorically be considered a prince, and was also mistakingly thought by the world at large to be the intended successor to Whitebeard himself. He was also captain of the Spade Pirates earlier, and later became commander of the second division of the Whitebeard Pirates.
 * Shanks is one of the Yonko and captain of the Red Hair Pirates. He also served as an apprentice under the late Pirate King and inherited Roger's Straw Hat which he later gave to Luffy.
 * Silvers Rayleigh has the epithet of "Dark King". He was also the right-hand of the late Pirate King.

Trivia

 * Hancock and Big Mom are the only known females to possess this type of Haki.
 * Ace and Luffy are the only known people with the will of the D. to possess this type of Haki.
 * Caesar Clown's attack Karakuni was mistaken for Haoshoku Haki by one of the Marines of G-5.
 * All currently known Haoshoku Haki users are or were pirates. No Marines or other citizens are known to use the ability.
 * This is the first form of Haki shown in the series, long before the concept of Haki was even mentioned.
 * It was shown in the first chapter when Shanks intimidated the Lord of the Coast from attacking him and Luffy.
 * Every canon character who possess this type of Haki has also shown to be able to use Busoshoku Haki.