Busoshoku Haki(Armament Haki), also known as Ryuo in Wano Country, is a form of Haki that allows the user to use their spirit to create, in essence, an invisible armor around themselves.
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.
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.
Busoshoku Haki can be 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. Swordsmen tend to use Busoshoku Haki to imbue their blades to prevent them from chipping.
Advanced Busoshoku can be emitted a short distance away from the user’s body without a medium. It can also be used in conjunction with the Hardening application.
This is first seen when Sentomaru deflected Luffy’s attack without making direct contact, though this was not explained until much later during the Wano Country Arc, when Luffy remembered how Rayleigh once used a similar technique.
Said use of Advanced Busoshoku involves channeling excess Haki from around one’s body through one’s fists. This allows the repelling of an enemy without making direct contact or using brute force. So far Kuzan, Sakazuki, Borsalino, Sentomaru, Rayleigh, and Hyogoro have been the only ones seen doing this.
An even higher grade of this technique causes the user’s Haki to reach inside the target’s body and destroy them from within.
This level has only been utilized by Rayleigh and Luffy, which the latter did unconsciously. Both used it to destroy collars that were designed to kill a person under certain circumstances.