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
The 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 child named Eri to produce it.

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. However, the drug can also be nullified by stopping the spread through the blood system by dismemberment, as Shota Aizawa was capable of stopping the drug by dismembering his own leg.

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 act 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 sold 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 re-emerging from the shadows.

It's unknown what happened to the antidote serum after the League of Villains ambushed Kai's police 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 one's Quirk Factor back to its undamaged state. After months of training, Eri managed to master her Quirk enough to restore Mirio Togata's Quirk Factor, meaning that she is now the only known cure for the drug.

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, 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, who was able to catch up with him before he could escape, Kai showcased the drug's effects by having Shin Nemoto 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 was confiscated by the Police Force.

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

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

Sometime later, Doctor Kyudai was successful in replicating the bullets. However, most of them were melted by X-Less during the raid on the Jaku General Hospital. When Tomura found about this, he lamented that Overhaul would be dissapointed if he were to learn of this.

Tomura takes into possession the two remaining bullets when he goes to fight the Heroes. He throws one towards Eraser Head, while he prepares to use the other on Endeavor. The bullet meant for Endeavor is exploded by Katsuki Bakugo before it can reach him. Eraser Head's bullet successfully hits his leg, which forces him to self-amputate it, so he doesn't lose Erasure. After that, all bullets were depleted completely.

Modification
Upon analysis, Doctor Kyudai realized that the drug was made from a person's Quirk, and by focusing on the original effects of the Quirk, created a one-time-only use "Rewind Drug", which he left with a hidden cache of equipment exclusively for his master, along with a voice recording of its abilities. Should it be used, all damage, even permanent, long-term damage they have previously received, would be reversed. However, there is no way to stop the effects of the drug, as such once used, the user's body will keep regressing until they eventually disappear. All For One was forced to use it during his battle against Endeavor, reverting the Villain back to his prime strength temporarily. When the user is forced to continuously exert themselves and use their Quirk(s) constantly, the effect of the drug is accelerated, as shown during All For One's battle with All Might, when the villain regressed from a young adult to a child at a much faster rate than normal.

Trivia

 * The Quirk-Destroying Drug could reference the "Mutant Cure" featured in "X-Men: The Last Stand".
 * Both are genetic weapons made from the power of a young child's powers (Eri and Jimmy, respectively).
 * The production of both are led by leading figures who saw their respective worlds' super-abilities as a disease (Overhaul and Warren Worthington II, respectively).
 * Both destroy the key genetic traits of their targets' super-abilities (a Quirk user's Quirk Factor and a Mutant's X-Gene, respectively).

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