Quirk-Destroying Drug

The Quirk-Destroying Drug refers to a substance that is capable of damaging the genetic mechanisms that compose someone's Quirk.

Overview
This drug was created by the Shie Hassaikai, a Yakuza organization led by Kai Chisaki, also known as "Overhaul", who was responsible for the conceptualization of the product itself, using the biological material of a young girl to produce it.

At the moment, the only known method to introduce the drug into someone is through the use of special bullets, red cartridges with hypodermic needles. Once the bullet hits the victim, the drug destroys the Quirk almost instantly. Alongside the drug, Kai also developed an antidote serum, which can nullify the effects of the drug and restore the victim's quirk.

With these two products in his possession, Kai planned to resurrect the glory of the Shie Hassaikai, and the Yakuza in general, by using the drug and the serum as a way to gather huge amounts of money as well as a tool to induce fear into the modern Quirk-dependent population. The drug was meant to be sold through the black market to villains, who would use them on Heroes and after a while, the yakuza would distribute a serum that would undo the damage done and restore the lost Quirks to the Heroes.

These products would be at a high price. Therefore, with the Villains having the Quirk-destroying bullets, the Heroes having the serum, and Eri in the middle, the Shie Hassaikai can monopolize control of the market, earning massive profit and eventually emerging from the shadows.

It's unknown what happened to the serum after the League of Villains ambushed Kai's escort. There's also no other known way to cure the effects of the drug, although Shota Aizawa believes that Rewind, being the genetic source of the drug, can revert back one's Quirk Factor.

History
The idea of a drug that could destroy one's Quirk was originally conceived by Kai, who tried to convince his boss, the former leader of the Shie Hassaikai, to allow the boss' granddaughter to have her body, which had manifested an unusual Rewind Quirk, used as a source of genetic material in order to cultivate said drug. Kai's plan was rejected by his boss due to its unethical nature, which led Kai to incapacitate his boss and take over the reins of the Shie Hassaikai.

By collecting genetic material from the girl's body (which was done through Kai's own Quirk, as he could destroy and restore her body infinitely), the Shie Hassaikai were able to experiment and develop a drug prototype that could damage one's Quirk Factor, the biological mechanisms behind the functioning of Quirks. The effects of this prototype, however, were only temporary, and thus were distributed solely for testing purposes. The prototype was primarily distributed in the form of injection bullets that could be fired into someone's bloodstream, disabling their Quirk for a few hours.

Requiring further funding, the Shie Hassaikai were able to strike an alliance with the League of Villains, who had an interest in the drug, having witnessed its effects during a skirmish with the Shie Hassaikai themselves. In a short amount of time, the drug was perfected, having its effects now become permanent. Additionally, a serum that could be used to restore the destroyed Quirks was created alongside the finished drug, being meant to be sold as well.

However, a rescue operation took place in the Shie Hassaikai's stronghold before Kai could commercialize the drug and the serum. During his battle against Mirio Togata, who was able to catch up with him before he could escape, Kai showcased the drug's effects by having one of his subordinates fire a drug-infused bullet into Mirio's body, permanently erasing his Permeation Quirk. This, however, didn't prevent Kai from being defeated and the drug confiscated by the police.

The League of Villains, however, successfully ambushed the police escort carrying Kai Chisaki and the case containing the product, retrieving the drug for their own ends.

In an attempt to replicate the bullets, Shigaraki asks Twice to duplicate them with his Quirk Double, but due to the lack of any data and accurate image of the drug, the bullets he creates do not last long and quickly fall apart. Twice apologizes to Tomura Shigaraki for not being to help mass-produce the drug as Tomura desired. Therefore, Tomura decides to find the doctor who created the Nomu and took care of All For One, with the hope that he could replicate the bullets.