A monster cell is a pulsating cell made of flesh and is roughly the size of a normal human hand. When a human ingests a cell, they undergo a physical and mental transformation that allows them to exceed their human limitations and turn into monsters. Monster cells were produced by Orochi through unknown means, and then collected and nurtured in large machines in the Monster Association Headquarters. The tanks were destroyed by Sweet Mask during the Hero Association's rescue operation.
Monster cells build upon the talents of the subject, but it comes at the cost of losing the potential to grow stronger. The monster cells also depend on the potential of the human itself to create monsters. The stronger the human, the stronger the monster transformation will be. This shows that humans with strong natural power are able to transform into more hideous creatures than weak people who still maintain a lot of their human appearance. Benpatsu, for example, only gained a third eye, sharp teeth, and elongated fingertips after eating a monster cell, and Rosie only gained an abnormal muscular physique and throbbing veins on his forehead with spinal protrusions sticking out his back. On the other hand, stronger humans such as Choze, Volten, Hamukichi, or Gouketsu have more monstrous and inhuman appearances.
When a human becomes a monster, they experience bloodlust, sadism, ecstasy, insanity and a loss of humanity, morality, and even rational thought, in which they will bend only on mayhem and destruction. Furthermore, the transformation effect is also very similar to when a human takes steroids, with some hosts showing intense pain upon ingestion. In exchange for this, they gain great strength that allows them to surpass their previous abilities. However, this is not considered a limit remover since becoming a monster is just putting a new Limiter on the transformed human. A particular weakness when a human becomes a monster through this method is that they will become extremely reckless due to the heightened bloodlust caused by the cell before learning to hone and control their new instincts. For example, newly transformed monsters often attempt to attack without gauging the power of their opponents. Benpatsu displayed this reckless behavior when he charged directly at Suiryu, only to be knocked out by one kick. Bakuzan also showed this behavior, believing that he is stronger than all S-Class heroes and monsters. This led him to attack Gouketsu, but the latter easily stopped his attack. The monster cell also brings out the worst of a human's personality when they become a monster. In the case of Bakuzan, not only did ingesting the cell make him more brutal and sadistic, but also showed true character he was hiding underneath his black heart: a bully who wanted to become strong just to prey upon the weak.
If the consumer has a weak will, they will lose their ego and intelligence, like Hamukichi and The Three Crows, and become little more than monstrous animals. Some stronger willed individuals are able to alternate between their human and monster forms. Such was the case with Haragiri, Gale Wind and Hellfire Flame, all of whom were experienced warriors with a mastery of mind and body. Apparently, eating more than one monster cell at a time can also either knock out the consumer or cause them to self-destruct, since the human body cannot handle the power of multiple cells all at once. In case of the former situation, the monsterification will be halted for some time until it can be completed. It was later revealed by Gyoro Gyoro that eating multiple monster cells is the same as consuming one, since monster cells simply transform abilities possessed by the original individual.
The monster cell must be eaten raw and willingly or else the cell will have no effect. For example, Speed-o'-Sound Sonic cooked a monster cell with wine after deciding to eat it and become a monster, reasoning that no one would eat something dirty and raw without knowing where it was from. However, this resulted in him not undergoing a transformation and instead having an upset stomach afterwards.
At some point in the past, Gyoro Gyoro and Orochi offered Gouketsu a monster cell to become a monster in exchange for his assistance to their cause.
Gouketsu gave the monster cells to his disciples, who became The Three Crows.
Gale Wind and Hellfire Flame ate monster cells and became monsters.
Haragiri, now a monster, explains to the Council of Swordmasters how monster cells turn humans into mysterious beings. He threatens the council to eat the cells or be killed, but is quickly defeated by Atomic Samurai.
When Gouketsu and the monsters crash the Super Fight tournament, they demand that the remaining fighters eat monster cells and transform or die. Rosie eats one of the cells and becomes a monster, but is quickly killed by Choze, who becomes a monster as well. Volten, Benpatsu and Hamukichi also eat cells and become monsters, though they are all defeated by Suiryu before long. Bakuzan tries to eat multiple cells at once in order to gain a stronger transformation, but apparently dies in doing so. It is later revealed that he did indeed transform and attacks Suiryu in his new form, almost defeating him before being killed by Saitama.
Speed-o'-Sound Sonic is given a monster cell by Gale Wind and Hellfire Flame in a bid to get him to join the Monster Association. He eats the cell after cooking it in wine, but it has no effect aside from giving him a stomachache, as cooking a monster cell renders its morphing abilities ineffective.
While invading the Monster Association Headquarters, Sweet Mask stumbles upon the machines responsible for processing the monster cells after being harvested from Orochi.
Sweet Mask concludes that in order to completely halt the production of monster cells he'd have to defeat Orochi, so he destroys the machines along with all of the monster cells within.
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