Monster Association

The Monster Association is a major antagonistic organization from the One Punch Man series, serving as the main antagonistic force in the Human Monster Saga. They are a terrorist organization, comprised of different monsters, made to counter the Hero Association. They seek to destroy the Hero Association, then create a world where monsters rule as the dominate species on Earth.

Monster Raid Arc
The Monster Association lead their first assault in one of the S-City. Metal Bat was in charge of guarding the city and thus had the task to take out the monsters. Eventually, the Elder Centipede emerged from the ground and battle Metal Bat. Despite Metal Bat's best efforts, he was unable to defeat the monster, as Elder Centipede continued on its rampage through the city.

Meanwhile, the Sludge Jellyfish attempts to kidnap Metal Bat's sister, Zenko, but was quickly dispatched by Garou, the Hero Hunter. Observing Garou and his raw talent, the Phoenix Man gave him an invitation to join their ranks, though Garou ultimately refuses, having no interest in joining their association.

Super Fight Arc
Gouketsu, along with the Three Crows, would later invade the Super Fight Tournament, where he would confront the martial artists and convince them into joining the Monster Association. Most of them joined, allowing themselves to be transformed into monsters, but were eventually beaten by Suiryu.

Gouketsu himself decides to dispatch Suiryu when he refuses to join the association, leaving him a bloody state. Bakuzan would then emerge from his transformation and attempt to show his skills to Gouketsu, only to be easily bested by him.

Gouketsu decided to leave and head for the upcoming meeting in the hideout, only to be killed off by Saitama.

Monster Association Arc
The Elder Centipede eventually confronts Bang and his brother, Bomb, who attempt to kill the monster. Despite their best efforts, the monster was too strong for them to beat. Genos decided to the matter into his own hands, but even his fire power was not enough. Eventually, King arrived to the scene to dispatch the monster. The monster charged towards him, only to be finished off by a serious punch from Saitama. Unknown to the Hero Association, it was Saitama that killed the monster.

Meanwhile, Garou would later inflitrate the hideout to rescue Busaiku, after being brutally beaten by Royal Ripper and Bug God. Garou eventually rescued Busaiku from being tortured by Royal Ripper. He would then later battle three monsters, before being punished by Overgrown Rover himself.

Garou was brought upon Gyoro Gyoro, who was interested in unlocking the full potential in him and perhaps make him Orochi's successor. Garou had no interest whatsoever, but Gyoro Gyoro would not take no for an answer. After struggling to keep him restrained, Gyoro Gyoro decided to let Orochi deal with him. Orochi would then proceed to beat Garou into a brutal state, though at the same time, avoid trying to kill him.

Members
The Monster Association is made up of a variety of different kinds of monsters, some being artificially created, some that were once human while others were naturally born. The Monster Association uses the disaster levels to determine the strongest members among their ranks.

The leader is selected based on their power and terrifying nature. Once selected, they are bestowed the Monster King title. Some monsters do not look up to the current leader as king, however and are willing to take the position themselves by challenging the current leader.

Leaders

 * Orochi (The current Monster King, Dragon +)
 * Psykos (Orochi's advisor and creator, Dragon)

Executive Members (Dragon Classes)

 * Black Sperm
 * Elder Centipede
 * Evil Natural Water
 * Fuhrer Ugly
 * Gouketsu
 * Gums
 * Nyan
 * Overgrown Rover

Demon Classes

 * Awakened Cockroach
 * Bug God
 * Devil Long Hair
 * Do-S
 * Eyesight
 * Face Ripper
 * Fist Fight Djinn
 * Free Hugger
 * G5 (Associate from The Organization)
 * Hundred-Eyes Octopus
 * Phoenix Man
 * Pureblood
 * Rafflesidon
 * Royal Ripper
 * Senior Centipede
 * Showerhead
 * Super Mouse
 * Three Crows
 * The Great Food Tub
 * Unihorn

Tiger Classes

 * Destrochloridium
 * Electric Catfish Man
 * Maiko Plasma
 * Marshall Gorilla

Unknown Classes

 * Benpatsu
 * Choze
 * Gale Wind
 * Gale
 * Goddess Glasses
 * Haragiri
 * Hellfire Flame
 * Hellfire
 * Junior Centipede
 * Rhino Wrestler
 * Rosie
 * Sludge Jellyfish
 * Venus Mantrap
 * Volten

Monster Cells
The Monster Cells are grotesque cells, produced by the Monster King himself, Orochi. They are made to be consumed by humans that are willing to join the Monster Association. Consuming one will transform the said person into a monster.

Depending on the person themselves, the Monster Cells may have different effects on each person. humans that are weak are likely to not gain much of a terrifying appearance, while those that are strong are more likely to gain terrifying appearances, in addition to powerful strength and abilities. For example, Benpatsu had consumed a Monster Cell and all it did was give him an extra eye, while Gouketsu consumed one, transforming him into a ferocious-looking beast.

The human body is also said to be able to only handle one Monster Cell. If a person were to consume more than one, the chances of them dying are likely. There are surprisingly exceptions to this, however, as Bakuzan had consumed a large amount of them and successfully transformed into a monster (albeit after being left unconscious).

The Monster Cells are also meant to be eaten raw and not cooked. If they are cooked, the cells themselves will fry, effectively killing them, meaning that there will be no transformation. Speed-o'-Sound Sonic attempted to eat one after cooking it and as a result, all it did was simply leave him with a stomach ache.