Busoshoku: Hanki

Busoshoku: Hanki (武装色·反鬼, Busōshoku: Hanki; lit. "Color of Armamnets: Reverse Demon") is a method of weaponizing the output of one's. Within the Haki Development System, it is classified as a Seventh Dan level technique due to its interactions with the surroundings — specifically its targets, causing certain types of reactions according to the user's wishes. While primarily used for "nullification", it's true use is emphasized in property change.

Overview
"W-W-What the hell is this?! How come I can't use my powers? Is this crap blocking it somehow?!"

- A Devil Fruit noticing a sudden "loss" of their powers.

In the Fourth Dan, Haki is able to nullify a Devil Fruit user's powers so long as they make contact; this makes it a suitable defense against such users, thus a requirement for Vice Admirals and high-level agents of the World Government to be at least at this level of Haki since most of their opponents are Devil Fruit users; in the same regard, it's not unheard of for pirates, mercenaries, swordsmen, and other fighters in the New World to be Fourth Dan users since most of their enemies and rivals are also Devil Fruit users. Of all the Dans in the Haki Development, most would strive for the fourth for this simplistic purpose.

However, as masters like Kurama have proven, Haki has a variety of purposes that transcend the mundane purposes of protection and enhanced sensory prowess; at the Fifth Dan, it's possible to project one's Haki outward and manipulate its nature in whatever way the user desires. The Busoshoku: Hanki makes use of that quality by altering the nature of whatever the projected Haki touches; the technique works by manifesting one's Haki as a dark ink-like substance that surrounds whatever the user touches, changing its properties accordingly to match the user's wishes. Typically users of this technique would take the "nullification" quality of Fourth Dan, turning off the Devil Fruit user's powers so long as the substance stays on the body. This gives them enough time to strike appropriately while worrying little about the target's resistance. Regardless, there were instances where the technique was used to weaken the opponent's defenses considerably by transforming their body into something that has a frail constitution.

The Haki substance is initially slow when users are introduced to the technique, thus contact is key — however, this no longer becomes an issue when users slowly master the technique or ascend to the next Dan in the system, the substance becoming much faster and more fluid. Busoshoku: Hanki works on entire bodies, thus can't be used on selective parts; in addition, the larger amount of Busō Haki used, the extended duration of the technique. It's said that Haki masters can gain immunity to the technique altogether due to using their own Haki to counteract the technique, nullifying it outright. While it can be used on multiple opponents at once or on a larger scale, it would require the user to have large amounts of Haki, something only a select few are gifted to have, and even then, it would still last for an instant because of the technique's mechanics.

Trivia

 * The term "Hanki" is derived from a use of Shunkō from the series, Bleach.
 * The overall mechanics of this variant is based on Estarossa's use of his darkness after absorbing two commandments from the series, Nanatsu no Taizai.