Monster Cells

Monster Cells (怪人細胞, Kaijin Saibō) are cells that can transform Humans into Mysterious Beings.

Overview
They have the appearance of pulsating sludge, and if humans inject those cells into themselves, they supposedly start to exceed their human limitations and turn into monsters.

The Monster Cells also depend on the base natural power of the human itself to create monsters. The stronger the human, the stronger the monster it creates. 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 sharp nails after eating a Monster Cell while Rosie only gained an abnormal muscular physique with protrusions sticking out his back. Stronger humans such as Choze, Volten, Hamakuchi, or Gouketsu have more monstrous and inhuman appearances.

When a human becomes a monster, they experience bloodlust, sadism, ecstasy, insanity and a loss of both humanity as well as morality and even rational thinking 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 old limits. However, this is not considered a limit breaker 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 blood-lust caused by the cell before learning to hone and control their new instincts. Examples of this recklessness is that a newly transformed human will often try to defeat an opponent many times stronger than themselves without confirming if they're stronger or not like Benpatsu charging directly at Suiryu only to be killed by one kick to the neck or Bakuzan believing that he is stronger than all S-Class Heroes and monsters to which he attacks Gouketsu without hesitation only to be stopped by the latter's superior strength. The Monster Cell also brings out the worst of a human's personality when they become a monster like in the case of Bakuzan, not only does the cell makes him more brutal and sadistic but also shows what he truly is from the depths of his black heart: a bully who wanted to become strong just to prey on the weak.

If the consumer has a weak will then they will lose their ego, like Hamukichi and The Three Crows, and become little more than monstrous animals. In the case of stronger willed individuals, not only do they gain a monstrous power boost, but can also will their transformations on and off in order to pass off as human through training and honing their new powers. Such was the case with Haragiri, Hayate of the Wind and Flame of the Hellfire, all of whom were experienced warriors with a mastery of mind & body.

The monster cell must be eaten raw and willingly or else the cell will have no effect. Apparently, eating more than one at a time, will knock out the consumer since the human body cannot handle the power of multiple cells all at once. In this situation, the monsterfication will be halted for some time until it can be completed. It is unclear if the consumption of too many of these cells will result in death, and it's also unclear if eating multiple monster cells will multiply the consumer's power or if it will remain the same as if the consumer ate one. Speed o' Sound Sonic cooked a Monster Cell in 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, he did not immediately undergo a transformation and instead had an upset stomach afterwards. This may indicate that cooking a Monster Cell renders it inactive.

Despite the ability to turn normal human into monsters, Gyoro Gyoro states that the cells on turn human into a quasi-monster state not true monster as the cells only improve the inherent strength of a natural body while failing to accelerate any further growth.

The "Monster King" Orochi is said to produce these cells.

History
At some point in the past, Gyoro Gyoro and Orochi captured Gouketsu and offered him to eat a cell and become a monster.

Gouketsu then gives the cells to his disciples, who become The Three Crows.

Gale Wind and Hellfire Flame ate the cells to become monsters.

Super Fight Arc
Haragiri explains using Monster Cells to turn into a Mysterious Being before the meeting of the Council of Swordmasters, and after consuming one himself and attempting to kill Atomic Samurai he is quickly defeated.

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.

Meanwhile, Nyan also uses Monster Cells to transform many of the prisoners in Smelly Lid Prison and takes them with him back to the Monster Organization hideout.

Speed-o'-Sound Sonic is given a Monster Cell by Hellfire Flame and Gale Wind in a bid to get him to join the Monster Organization. He eats the Cell after cooking it in wine, but it doesn't seem to do anything besides give him a stomach ache.