Anti-Kibutsuji Drug

The Anti-Kibutsuji Drug is a unique drug with the main purpose of defeating the Demon King, Muzan Kibutsuji.

Overview
The Anti-Kibutsuji Drug is a special four-stage drug that was created by the combined efforts and medical expertise of Tamayo, Yushiro and Shinobu Kocho with the main purpose of annihilating, Muzan Kibutsuji.

It was developed after the attack on the Swordsmith Village and used in the final battle in the Infinity Castle. Before Kagaya Ubuyashiki and his family sacrificed themselves in a suicide attack to stun Muzan, Tamayo assimilated some of the drug herself, so that if the likely possibility that Muzan would absorb and assimilate her body, the Demon King would also take in the drug on accident.

Every stage of the drug was meant to account for every one of Muzan's strengths, advantages and abilities.

Stages
There are four stages of this drug, each of which is activated sequentially after the previous stage has taken its effect on Muzan Kibutsuji.


 * 1) Human Conversion Stage: The first stage of the drug, which was created even before the need arose by Tamayo, which was given to Nezuko Kamado. This stages effect is to convert Muzan back into a human. Muzan started this stage to take effect after absorbing Tamayo's arm, which contained the drug. The ingredients utilized in the making of this stage are quite vague, but Tamayo hinted at the use of the blood of powerful demons, which have a high concentration of blood from Muzan, and the blood of Nezuko Kamado. After she teamed up with Shinobu Kocho, the drug was improved and the effects was drastically enhanced. However, Muzan was able to completely dissolve and nullify it, but in return, accidentally triggered and allowed the other stages of the drug to begin to take effect.
 * 2) Biological Aging Stage: The second stage of the drug which Muzan assimilated after absorbing Tamayo and began showing its effects and significantly affecting the Demon King after Tanjiro Kamado started engaging in combat with him in their final confrontation. This stage was proposed by Shinobu Kocho but the ingredients utilized were unknown. The intended effect of this stage is to age Muzan Kibutsuji over 50 years in a single minute, which to a demon, especially the Demon King, would be practically meaningless since they are immortal, but in Muzan's case, who is the oldest being in existence, any aggravation of his biological age would be extremely injurious to his health despite his nigh-absolute regenerative abilities. This effect was able to physically age Muzan by over 9,000 years after being in effect for roughly 3 hours, making his total biological age over 10,000. This stage of the drug is arguably the most important stage as it drastically decreased Muzan's unparalleled physical capabilities and condition, even nullifying his unlimited and boundless stamina, to the point where the injured Hashira and Demon Slayers could keep up with his attacks and movement speed, albeit barely.
 * 3) Splitting Prevention Stage: The third stage of the drug is created by Tamayo herself after witnessing Yoriichi Tsugikuni nearly succeed in killing Muzan Kibutsuji, who only managed to barely escape by splitting himself into hundreds of pieces. Thus, Tamayo, through unknown means, created a special compound that would prevent Muzan from splitting himself up and escaping during the final battle. This stage of the drug was always dormant after Muzan absorbed it, but he only realised it after he attempted to split himself, which Tamayo anticipated.
 * 4) Cellular Destruction Stage: The fourth and final stage of the drug which was meant to finish Muzan after he was weakened by the previous 3 stages. This stage of the drug is meant to cause Muzan's cells to slowly deteriorate and perish, which was facilitated by the aging stage of the drug that weakened his immortality and grasp on his own body. This stage had an extremely profound and significant effect on his regenerative abilities, to the point that a non-crimson red Nichirin Sword was able to cause serious damage to his body, as well as his healing process to slow down from healing instantaneously to taking several seconds to heal even the most minor of wounds, even being unable to heal some more serious injuries.

In the end, all four stages combined managed to delay the fight till sunrise arrived while also constantly weakening Muzan, allowing the Demon Slayers to push him to a corner, prevailing against the Demon King after 1,000 years of war and bloodshed.