Busoshoku Haki

Busoshoku Haki is a form of Haki that allows the user to use their spirit to create, in essence, an invisible armor around themselves. Notably, this form of Haki can be used to injure Logia Devil Fruit users.

Similar to Tekkai, this Haki allows the user to defend against attacks that would otherwise cause them harm.

This form of Haki is by far the most common of the three.

Background
It was used by Garp for the first time against Luffy to bypass his Devil Fruit power and hurt him in Water 7.

Later, on Sabaody Archipelago Sentomaru used a Haki-imbued attack against Luffy, which made the Straw Hats think he was a Devil Fruit user due to their lack of knowledge about the ability. Afterwards, Rayleigh used Haki to stop Kizaru and even touched his Logia body.

The first time the Kujas named this ability "Haki", they were shown wielding Haki-imbued arrows. During the Marineford War, Haki was also used many times by Marines and pirates alike.

It was shown that Haki is very common in the New World, unlike in Paradise where Haki users are fairly rare and almost nobody knows about it. It is later explained by Rayleigh once he begins training Luffy.

Usage
Naturally this "invisible armor" can also be used as a weapon to strike others with. It can be used to augment the user's own physical attacks, projected ahead of the user as pure brute force, or imbued into weapons to increase their effectiveness. Any attack and/or weapon enhanced by Busoshoku Haki hits immensely harder than it would normally without it. The Kuja's Haki-imbued arrows, for example, are capable of penetrating solid stone. Swordsmen tend to use Busoshoku Haki to imbue their blades to prevent them from chipping.

This type of Haki also has the ability to bypass the powers of a Devil Fruit user whose body has been altered by their fruit in any way, touching the "substantial body" beneath whatever protection the fruit provides, such as the case of body-altering Devil Fruits such as Logia or Paramecia users. However, unlike Seastone, it does not actually nullify a user's Devil Fruit powers, so it still allows them to defend themselves, and continue fighting normally. By the same effect, this Haki can also be used as a higher level of offensive power, more so than just bypassing a foe's Devil Fruit powers.

A heavy concentration (or coating) of this Haki can be used to harden parts of one's body, turning the area black, thus making the user denser and more formidable, and giving their attacks greater power. As demonstrated with Vergo and later Pica, it is possible for one to coat their entire body in Busoshoku Haki. Although Luffy, the first in the mainstream storyline to demonstrate this effect, named the technique as Busoshoku: Koka, Donquixote Doflamingo and Roronoa Zoro are the only people to use this name for the technique.

Techniques
 * Busoshoku: Koka - This technique somehow uses Busoshoku Haki to harden the body (or parts of it) and objects which the user is holding. It was first seen used by Monkey D. Luffy, in combination with Gear Third, in order to use his "Gomu Gomu no Elephant Gun" against the Kraken. When a body, body part, or object is hardened, it becomes black and has the same or stronger effect than the invisible application of Color of Armaments.


 * Atama Buso - Used by Luffy, he imbues his forehead with Busoshoku: Koka to harden it.
 * Gear Second Buso - Used by Luffy, he imbues his fist with Busoshoku: Koka while under the effect of Gear Second to harden it, which also seems to have the added effect of igniting it.

Drawbacks
When two opponents using Busoshoku Haki clash, the one with the stronger Busoshoku Haki is more likely to be victorious. This can make it difficult to overcome an opponent's Haki in a brute force battle. However, it has been shown that superior Busoshoku Haki is not an absolute defense, and can be overcome as long as the other opponent has superior physical strength, as shown when Luffy was able to damage Katakuri with Gear Fourth.

Significant physical force can still overcome this defense; for example, Luffy struck Boa Marigold hard enough with Gear Second techniques that it sent her reeling.

Busoshoku Haki is subjected for depletion if used for long periods of time as Busoshoku Haki is proportional to the amount of spirit life force an individual possesses and its stamina limitance. After the threshold is reached, the user will be unable to use it for a certain amount of time. It appears that the more Haki is used, the faster it will deplete. The users are unable to use Haki until they regain their stamina to use it again.

Interactions with Devil Fruit Users
Devil Fruit Users are capable of using Haki themselves. This allows them to resist lesser Haki-imbued attacks, yet it is still possible to pierce through their defense if the attack is stronger. Busoshoku Haki works independently of the user's Devil Fruit if they have one, as shown when Luffy uses it to enhance his Gear Third attack underwater. This type of Haki may not be equipped on attacks that are not from one's body except for weapons, but can be equipped on anything generated by one's Devil Fruit (like Kizaru's laser beams, Marco's flames or any other attacks based on Devil Fruits). Charlotte Katakuri has shown that one does not even need to be in direct contact with said substance to imbue his Busoshoku Haki into it.

Using Busoshoku Haki attacks on the element controlled by Logia type Devil Fruit Users will allow the attack to harm the "substantial body" of the opponent, since the element is essentially an extension of their body. However, this type of Haki will not harm the "substantial body" when used on elements controlled or produced by Paramecia type Devil Fruit users, since it is not actually part of their body (as demonstrated by Trebol). If the Paramecia type Devil Fruit affects the user's body itself (Luffy, for example is literally made of rubber) the user will also be harmed by a Busoshoku Haki imbued attack.

Haki's capability to counter Devil Fruit Users extends past Logia types, and lets them defend against certain Paramecia effects. However, this is dependant on the strength of the Haki, as Law bypassed Vergo's Haki and bifurcated him easily by using his Devil Fruit technique, although it is unknown if Law used his own Haki to overcome Vergo's Haki but it is similar to how Akainu was seemingly able to nullify his attacker's Busoshoku Haki.

While Haki can bypass the devil fruit's defense, it doesn't negate the devil fruit, like Sea-Prism Stone does. Instead it "forces the devil fruit user's body into solidarity".

Trivia

 * Busoshoku Haki is similar to Tekkai. The difference between them is that Tekkai depends on the user's physical strength, but Busoshoku Haki depends on the user's willpower and life force. Also, while Tekkai is more inclined to defense, with the user usually being immobile while tightening their bodies, Bushoshoku Haki does not have such a drawback. On the other hand, overusing Busoshoku Haki consecutively may drain the user for a period of time, a weakness not known to Tekkai.
 * Busoshoku Haki users can still utilize their Devil Fruit powers, despite Haki being able to negate Devil Fruit defenses. This is because Haki only allows the user to bypass defenses provided by other peoples' Devil Fruit powers. In fact, Devil Fruit users can use the two abilities together to make one or the other more effective.
 * Busoshoku Haki is the most used type of Haki in the series so far, as it is the most related attack and defense power in combat.
 * Although the black skin effect of its usage is very common after the timeskip, it was not seen at all before, not even during Summit War Saga, where many pirates and Marines of the highest level, who have been confirmed to be using Haki, participated. The reason for this is currently unknown (though this was most likely done, so that fans wouldn't be confused by this change in color before Haki was fully explained by Rayleigh at the end of the first half of the series).
 * When Luffy uses Gear Fourth form, his Busoshoku Haki takes the shape of tribal flames on his body; this is the first example of Busoshoku Haki forming a recognizable pattern. Big Mom's Haki takes the same form, though it's limited to her arm. When Katakuri performs his Zan Giri-Mochi attack, his Haki also becomes flame-shaped.
 * There is an extra color tone to the black skin effect in some characters when this Haki is used at its maximum (pinkish red for Luffy in Gear Fourth, purple for Vergo and Pica when used all over their bodies, and blue for Katakuri while using Zan Giri Mochi).
 * In the anime, Busoshoku Haki has more visual effects when used at its fullest than just the black skin. These effects are a green flame-like aura (in Sai's case), and a steam-like aura (black and purple for Zoro, Fujitora, and Marines; purple for Kin'emon; and pink for Kanjuro).
 * Jinbe is the first fishman shown to use Busoshoku Haki.
 * Pekoms is the first member of Mink Tribe shown to use Busoshoku Haki.