Busoshoku Haki (Armament Haki) is a form of Haki that allows the user to use their spirit to create, in essence, an invisible armor around themselves.
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.
Busoshoku 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 Busoshoku 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.
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.
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.
Busoshoku Haki works independently of the user’s Devil Fruit if they have one. It may not be equipped on attacks that are not from one’s body except for weapons, or 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).
This type of 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. 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.