After its base is raided by the Hero Association, the Monster Association collapses.
The Monster Association consists of around 500 monsters, about 30 of which are disaster level Demon or higher. The Boss (ボス, Bosu) is the Monster King, Orochi, although the organization is actually overseen by the Military Advisor (参謀, Sanbō), Gyoro Gyoro (who is actually a puppet of Psykos, the creator of Orochi). Directly below them in ranks are the Executives (幹部, Kanbu), also known as Cadres (referred to as Leaders in the Viz Media), who are specially promoted by Gyoro Gyoro.
The Monster Association is founded on the belief that one monster alone will eventually fall, but all of them together can defeat the heroes. Many monsters believe that they are stronger than the heroes; Gyoro Gyoro has to remind them that this will be their downfall, though sometimes he uses this to his advantage.
The goal of the Monster Association is to end the Hero Association and create a world of monsters ruled by Monster King Orochi. However, most members only joined forces for the former and do not view Orochi as their king, but they do acknowledge his great power. The plan is to lure the Hero Association into a decisive showdown. There are three primary objectives that are required to make this plan feasible:
- Strike fear to the human population through monster rampage.
- Kidnap important hostages that are vital to the operation of the Hero Association.
- Transform powerful humans into monsters to boost the military strength of the organization.
New monsters are recruited by offering monster cells to strong humans, such as the martial artists of the Super Fight. Whoever eats the cells transforms into a monster, but it has to be eaten raw. Typically they are eaten one per person, as eating more can result in failure and death. A Dragon-level monster born from the monster cells is considered a "huge success." Other monsters can join the Monster Association by going to the base beneath Z-City and meeting with Gyoro Gyoro.
Monster Association's hierarchy is based on disaster level. Gyoro Gyoro assesses each member and assigns them a level. The executives are monsters that earned the level of Dragon. Lesser monsters have the utmost respect for the executives and fear provoking them.
Outside of the executives, there are at least three other monsters that have reached the level of Dragon: Gale Wind and Hellfire Flame are Demon-level in their human forms, and Dragon-level in their monster forms; they are executive candidates but claim to not care for their ranks. The monster acting as middle management, Phoenix Man, was originally Demon-level, but became Dragon-level upon resurrecting.
Members are not permitted to kill each other as it will weaken the organization, although Gyoro Gyoro admits that this is difficult to enforce. Many of the monsters are hostile towards each other. Presumably, this only applies to strong monsters since Gyoro Gyoro himself treats weak monsters as expendables and sometimes feeds them to Orochi. Even Demon-level threat monsters are not safe: Orochi ate Awakened Cockroach for losing, and Gyoro Gyoro used Rhino Wrestler as bait. Monsters with useful abilities, such as Do-S, are somewhat protected.
The Monster Association is an unknown group founded by Psykos and Orochi. The headquarters of the Monster Association was previously the home of the Subterranean People who were all slaughtered. Psykos created the meat puppet Gyoro Gyoro to control the organization while her real self remains hidden further underground.
In S-City, Venus Mantrap and Junior Centipede attack a sushi restaurant where the Hero Association benefactor Narinki and his son Waganma are dining. Metal Bat, who is guarding them, easily defeats both monsters. Rafflesidon and Senior Centipede then emerge from the ground. Narinki and Waganma are knocked out by Rafflesidon's gas. Metal Bat fights the monsters, but is slightly sedated by the gases the monster released. He manages to break out of its control by smashing his head, arousing his fighting spirit, and kills both monsters with one strike each.
Moments later, Elder Centipede, a Dragon-level threat, emerges through the ground. Elder Centipede ignores Metal Bat's attacks and goes for Narinki. As Elder Centipede is about to catch the sponsor, he is struck by Metal Bat, and tries to shake Metal Bat off by maneuvering around the city. Elder Centipede manages to launch Metal Bat away and continues to rampage through the city. Just as Elder Centipede corners the heroes Pineapple and Mohican, who are carrying Narinki and Waganma, it is attacked by Metal Knight's missiles. Phoenix Man, Sludge Jellyfish and Rhino Wrestler then corner the heroes and demand the sponsor. After Garou finishes his fight with Metal Bat, Phoenix Man and Sludge Jellyfish discuss how they could use him to their advantage.
After Metal Bat was knocked out by Zenko, Sludge Jellyfish tries to take her as a hostage but was struck by Garou. The monster was able to regenerate itself and Phoenix Man attempts to invite Garou into the Monster Association. Garou rudely declines by ripping up the invitation card. The monsters say they'll overlook Garou's rude behavior this time and promise that they will meet again if Garou continues to carry out his hero hunt. Meanwhile, Rhino Wrestler fights and defeats Pineapple and Mohican, and takes Waganma hostage. The monsters then retreat after explaining how fighting Metal Knight would only be in vain for them. The monsters retreat, but Metal Knight manages to stick to Elder Centipede and follow them to their base.
The Monster Association orchestrates simultaneous attacks many cities:
- W-City: Heavy Kong fights against Marshall Gorilla.
- D-City: Lightning Genji fights Maiko Plasma and Electric Catfish Man.
- I-City: Hundred-Eyes Octopus wreaks havoc on the city
- Y-City: Eyesight wreaks havoc on the city.
- V-City: Pureblood wreaks havoc on the city.
- Unknown city: Do-S and Free Hugger wreak havoc on the city.
Fist Fight Djinn is fighting the A-Class hero Smile Man, when Bang arrives at the scene. Fist Fight Djinn attempts to attack Bang, but is mercilessly killed by the S-Class hero. Meanwhile, Do-S manages to defeat many heroes and is confronted by Fubuki and her Blizzard Group. Marshall Gorilla manages to defeat Heavy Kong and tells the civilians to spread the word about him. Maiko Plasma and Electric Catfish Man fight Lightning Genji and manage to hospitalize him. Hundred-Eyes Octopus keeps devouring buildings and easily manages to defeat Death Gatling, Butterfly DX and Bone. Eyesight manages to poison Ecolo G and other heroes and fight Studless. Pureblood manages to kill many civilians.
Many monsters that appeared in C-City are defeated by Genos and Face Ripper later encounters the hero. Face Ripper manages to damage Genos, but is caught off guard and killed. In the same city, Awakened Cockroach defeats many heroes and meets Genos in an alley. On TV, it is reported that Lightning Genji is defeated by the monsters. While Awakened Cockroach manages to have the upper-hand in the fight against Genos, the monster is caught in an adhesive trap, though he manages to escape before being struck by Genos' blast.
Do-S manages to transform all of the Blizzard Group members into her love slaves. While Fubuki is preoccupied with the love slaves, Do-S manages to pierce through her telekinesis and whip Fubuki. Do-S manages to hit Fubuki multiple times and believing that she has Fubuki's mind under her control, she reveals her plan of using Fubuki to lure Tatsumaki out. However, Do-S is sent flying and attacked by Fubuki. Angered, she decides to kill Fubuki. When the area begins to rumble, a monster falls in front of Do-S and warns her about Tatsumaki. Frightened, Do-S orders her love slaves to distract Tatsumaki, while she escapes.
The Monster Association continues their attack. Sweet Mask's concert is interrupted, the hospital where Mumen Rider, Tanktop Master and the Tank Topper Army are staying is attacked, and Nyan breaks into Smelly Lid Prison in search of Puri-Puri Prisoner. At their hideout, Gyoro Gyoro and Monster King Orochi are discussing their invasion and Gouketsu's whereabouts. They manage to defeat S-Class heroes like Genos and Metal Knight, whose robot is hanging in chains severely damaged and captured at the Monster Association's Headquarters.
Despite their success, many monsters are killed by S-Class heroes. Drive Knight easily kills some members and interrogates the last living one about the invasion. Child Emperor uses an Underdog Man No. 22 to fight Eyesight, but the monster is later eaten by Pig God. Hundred-Eyes Octopus's destruction forces an A-Class heroes team to be assembled, but is later blinded by Flashy Flash, and crushed into a ball by Tatsumaki. Gyoro Gyoro worries slightly about the power of the S-Class heroes, especially Tatsumaki, but Orochi reassures Gyoro Gyoro that it will not be a problem as they can make as many Mysterious Beings as they please.
During the Council of Swordmasters meeting, called by Atomic Samurai to help hunt Garou, one of the Council members, Haragiri, revealed himself to be a monster by ingesting some Monster Cells. Haragiri orders the others to transform into monsters and attacks Atomic Samurai due to orders from the Monster Association, but he is killed by Atomic Samurai. Through his robot, Metal Knight spies on the discussion between Gyoro Gyoro and Orochi. When asked about their goals, Gyoro Gyoro simply tells him to watch and wait. At the hospital, the Tank Topper Army fights the monsters and protects the patients and staff members. Mumen Rider struggles against Gyoffrey, but is saved by Tanktop Master.
Gouketsu and The Three Crows interrupt the closing ceremony of the Super Fight Martial Arts Tournament in an attempt to transform all of the martial artists into monsters. Gouketsu brings one Monster Cell for each martial artist. Rosie, Choze, Benpatsu, Hamukichi and Volten all join the Monster Association, though Rosie is quickly killed by Choze. While the other martial artists are frightened, Suiryu decides to rebel against them. After Suiryu manages to defeat all of the transformed martial artists, Gouketsu is impressed and offers Suiryu an executive position in the association, but he is rejected and then proceeds to strike Suiryu down.
Gouketsu then toys with Suiryu, stomping and punching the martial artist, giving him only three seconds to recover. In an attempt to win, Suiryu asks Bakuzan, the last standing martial artist for aid, but Bakuzan instead eats several Monster Cells and collapses on the floor, seemingly dead. Gouketsu then orders The Three Crows to finish off Suiryu, but the Crows are later killed by Suiryu in a team effort with Lightning Max and Sneck. After the Crows are killed, Gouketsu is challenged again by Sneck and Lightning Max. He easily defeats them and when Suiryu tries to leave the stadium, Bakuzan wakes up from his transformation and blocks his entrance. Bakuzan then sadistically strikes Suiryu on the ground.
Gyoro Gyoro uses its spy drones to watch the chaos in the city and confirms that their first objective is successful, as they also capture Waganma, the son of an influential Hero Association sponsor. Phoenix Man notes they lost many monsters and asks for reinforcements, but the request is denied. The monster notes that Nyan managed to recruit several new members and Gyoro Gyoro orders Gouketsu to leave the stadium, but Gouketsu refuses. Gouketsu questions Bakuzan and tells him to go with him to the Monster Association hideout, but Bakuzan refuses and fights Gouketsu. Gouketsu then warns Bakuzan about his attitude and informs him of Orochi, the strongest being on Earth. He then gives Bakuzan a firm warning as he leaves the stadium for Z-City, the meeting point of the Monster Association.
As Gouketsu is heading towards the hideout, Saitama finds him and they fight off-screen, with Gouketsu's decapitated head flying into the stadium and landing in front of Suiryu. Free Hugger manages to injure Puri-Puri Prisoner, but the monster is squeezed to death by the hero.
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Elsewhere, Gale Wind and Hellfire Flame attack Speed-o'-Sound Sonic who is training in the forest. As they are discussing, the ninjas reveal they have joined the Monster Organization and with their advanced training managed to retain their human form. Gale Wind and Hellfire Flame reveal they joined the Association to rule the world, and more specifically to kill Flashy Flash. They ask Sonic to join them. Sonic initially refuses, but when Gale Wind demonstrates his speed after his transformation, Sonic is persuaded and takes a Monster Cell. Later Sonic cooks the Monster Cell into a gourmet meal; however, he doesn't transform into a monster, but has an upset stomach afterward instead.
In W-City, Marshall Gorilla still wanders around the streets, feeling unsatisfied by his threat level being Tiger and wants the Hero Association to change it to Demon. Zombieman is following the monster and wants to gather as much information about the organization and its hideout. Marshall Gorilla then spots someone on the streets and is surprised to see Armored Gorilla, who is shopping for groceries. Marshall Gorilla feels angry that Armored Gorilla "sided" with the humans and attacks him only to be easily defeated.
In A-City, Destrochloridium takes control of an association member and takes one of Gyoro Gyoro's drones so they could declare war on them. He kills one of the association members before being killed by Superalloy Darkshine. In their headquarters in Z-City, Gyoro Gyoro gives a speech to the monsters and Orochi decides to eat Awakened Cockroach for failing to kill Genos. Orochi wants to kill Do-S as well, but decides not to, since Gyoro Gyoro stated her powers are useful. When they noticed Gouketsu's absence, Gyoro Gyoro uses one of its drones, only to find Goukestsu's headless corpse.
While Garou is being attacked by Genos, Bang, and Bomb, some of the Monster Association's members arrive to save him but are swiftly beaten. Phoenix Man swoops in and saves Garou while Elder Centipede causes mayhem.
Elder Centipede then attacks and overwhelms Bang, Bomb and Genos, before being killed by Saitama. Meanwhile, Phoenix Man escapes with Garou. At the Monster Association base, Garou wakes up and encounters Monster King Orochi, Gyoro Gyoro, Royal Ripper, Bug God, G5 and many other monsters. Garou is offered an executive position under the condition that Garou kills a hero within a day. He leaves to do so while being tailed by Royal Ripper and Bug God. After tailing him, Royal Ripper and Bug God deem Garou to be unworthy and attack him. Due to Sludge Jellyfish holding Tareo hostage, Garou is stabbed to the point of unconsciousness. Tareo is taken back to the Monster Association and thrown in the same cell as Waganma. Gyoro Gyoro reprimands Royal Ripper as the Hero Association will attack soon and they need to be focused on the coming battle.
Various monsters such as Maiko Plasma and Electric Catfish Man are discussing their battles. Phoenix Man, after discussing Orochi's process of monsterization with Gyoro Gyoro, contemplates on becoming the Monster King. Do-S is training the mind-controlled mercenaries. Hellfire Flame and Gale Wind are training against each other. Pureblood drains another monster dry of blood for not walking out of his way. Devil Long Hair is talking with another monster while Pureblood looks on in disdain. Nyan is chatting with the monsterized prisoners from the Smelly Lid Prison. G5 collects Metal Knight's head and throws a monster at The Great Food Tub. Garou wakes up from his injuries and heads towards the Monster Association base. Meanwhile, Child Emperor nearly finishes locating the Monster Association base.
Garou kills any monsters he comes across and saves Tareo from Royal Ripper by killing the monster. Garou eats a monster's arm to replenish his lost blood. They come across Overgrown Rover and quietly escape only to run into Showerhead, Unihorn and Super Mouse. Garou fends them off but attracts Overgrown Rover's attention. The three monsters are killed by Overgrown Rover and Garou engages it in battle, but he is blasted to the floor Gyoro Gyoro resides. Garou is immobilized by Gyoro Gyoro's psychic power, after which Gyoro Gyoro proceeds to tell Garou the secret of overcoming one's Limiter by repeatedly experiencing death and overcoming it as was the case with Orochi. Gyoro Gyoro plans to use Garou to create the ultimate monster, but Garou refuses and plans to hunt Gyoro Gyoro and destroy the Monster Association. Orochi then intervenes and overwhelms Garou in their battle, knocking him out.
Later, the Hero raid on the Monster Association base takes place. Several monsters, including Evil Eye and Rhino Wrestler are sent to kill the invading heroes, but they are killed.
Flashy Flash encounters and kills Hellfire Flame and Gale Wind, but Flashy Flash loses his Locator during the battle and follows a random path. Child Emperor finds the decommissioned Metal Knight unit, then proceeds to search the robot's database and manages to find the location to Waganma's cell. As he leaves, Child Emperor encounters Phoenix Man and shreds the monster. Child Emperor saves Waganma and the hero kills Sludge Jellyfish. While leaving, they run into G5 whom they retreat from. During their retreat, they come across Phoenix Man's corpse. Sudenly, Phoenix Man's resurrection activates and he returns fully healed and stronger, with a higher disaster level of Dragon. Child Emperor is forced to summon his ace weapon, Brave Giant to battle. Their battle resumes with Phoenix Man getting stronger with each new resurrection. Phoenix Man reveals his new ability to resurrect the dead to fight and brings back the Subterranean People, Gale Wind and Hellfire Flame to fight. While the Subterranean People are incinerated, Gale Wind and Hellfire Flame are brought back to life after being electrified by Brave Giant. Child Emperor defeats Phoenix Man by sneaking Tickle-Tickle Bug 1 into his suit and causing the monster to rip apart his costume. With his powers lost, Child Emperor ends Phoenix Man.
Zombieman is killing any monsters he encounters and is led to Pureblood. After a grueling 30 minutes, Pureblood lays defeated. After defeating the remaining monsters, Zombieman leaves to carry on his mission. Pig God encounters and devours The Great Food Tub. The Disciples of Atomic Samurai encounter Do-S and the mind-controlled Narinki's Private Squad. Their battle is interrupted by Sweet Mask who takes control of the battle and tells the swordsmen to leave. As they leave, they encounter Pig God in the middle of digesting The Great Food Tub and decide to take another route. The swordsmen encounter Devil Long Hair and they battle. Sweet Mask kills the mind-controlled mercenaries and defeats Do-S.
Meanwhile, Puri-Puri Prisoner is battling some monsters, and then encounters the demon-level monster Baquma who can absorb monsters and use their powers. Despite this ability, the hero defeats the monster. The Smelly Lid Prisoners who were monsterfied confront Puri-Puri Prisoner to kill him, but the hero mistakenly believes they were bewitched by the Monster Association. Nyan attacks Puri-Puri Prisoner, but is put off by his strange personality and opts to run away. Puri-Puri Prisoner, however, gives chase but accidentally kills almost all of his fellow inmates.
Superalloy Darkshine is in battle with Bug God, resisting every one of the monster's attacks without retaliating. When Bug God mocks Superalloy Darkshine for this, the hero counters by stating the monster fell below his expectations and adds that he, Tatsumaki, and King are not to be angered. Bug God, offended by this, eventually assumes a more powerful form and attempts to defeat the hero, only to be crushed by a single of his attacks.
Meanwhile, Saitama encounters Overgrown Rover and easily fends the monster's blast and sends it flying with a single punch, causing a loud tremble that goes noticed by the heroes and the executives alike. Nyan, shocked by this, attempts to sneak attack Saitama, only for his attack to have no effect. Realizing the risk of fighting Saitama, Nyan quickly retreats.
In another place of the headquarters, Tatsumaki encounters Gyoro Gyoro, who boasts the organization's goal to create a world of monsters and engages her in combat, first by trying to overpower her by crushing her in compacted debris, to no avail. Tatsumaki rips a chunk of Gyoro Gyoro's body with a single psychic blast, prompting the monster esper to assume a more powerful form and trying to pin her with 300-fold gravity, which proves to be in vain as Tatsumaki rips Gyoro Gyoro's body and realizes said body was a meat puppet. Desperate, Gyoro Gyoro pleads to Orochi for help, only to his cry for help to go unheard as Orochi's attention is diverted to Saitama.
Orochi mocks Saitama and tries to attack him, only for his blow to be intercepted, and for him to finally realize the previously felt tremble was Saitama's doing and thus recognizing him as a dangerous threat to be eliminated. Saitama crushes one of Orochi's appendages and later easily dodges one of his fire blasts. As Overgrown Rover is nearby recovering from the previous impact, Orochi commands him to kill Saitama, only for the canine monster to flee in fear upon recognizing who overpowered it. As Saitama effectively admits he beat up Overgrown Rover, Orochi immediately realizes he was behind Gouketsu and Elder Centipede's deaths, and feeling a long-unfelt excitement, Orochi unleashes all his power on Saitama, while boasting about it, which annoys Saitama. The ensuing battle's ripples are felt throughout the headquarters and the entirety of Z-City, as Atomic Samurai fends one of Orochi's stray lightning blasts, Bang and Bomb guard from another, the city landscape is leveled, and a large subterranean area is pierced in a clean line from another blast. Saitama easily rushes towards Orochi and defeats him with a single normal punch. As he suffers defeat, Orochi wonders about feeling terror for the first time.
Gyoro Gyoro, through one of her drones, finds Orochi's lifeless head, and tries to find the one responsible in order to devise countermeasures. Gyoro Gyoro spots Saitama, who she doesn't recognize, for a moment only for her drone to be crushed by him. Tatsumaki finishes what is left of Gyoro Gyoro's puppet body, only for a humanoid figure to emerge from its remains, who Tatsumaki recognizes.
Elsewhere, a monster posing as Waganma tries to fool a fearful King into following him to a trap, only to melt in fear itself upon mistaking King's meek words as an intimidating stance.
The battle between Atomic Samurai's disciples and Devil Long Hair eventually ends as the monster's weak point, his hair, is cut short. Elsewhere, Atomic Samurai easily cuts through several monsters and eventually encounters Sword Devil Executioner, whose blows are easily blocked by the hero and gets killed. G5 engages Atomic Samurai after collecting data in order to copy his abilities. Despite being initially evenly matched, Atomic Samurai cuts through G5, though the robotic monster's core is intact and eventually escaped. As Black Sperm is chasing Tareo, he is stopped by Atomic Samurai through a slash.
As both hero and monster mock each other, Black Sperm's body bulges over and forms a large, muscular arm, only to be cut by Atomic Samurai, but another Black Sperm generates from the severed arm and the main one sprout multiple heads which are cut by the hero; however, multiple Black Sperms grow from the severed parts and even more after being cut again, which eventually begin overwhelming Atomic Samurai blow-by-blow. The hero briefly regains ground by focusing his atomic slashes and disintegrating every Black Sperm, but while trying to escape he is met by a bulky Black Sperm who overwhelms him again. After the Black Sperms split again, Atomic Samurai slices the ceiling to crush them.
Zombieman notices the rumble caused by Atomic Samurai's slashes and shows concern about the status of the battle, when suddenly Homeless Emperor sneaks on him and attacks him with a barrage of light blasts. As Zombieman survives the executive's attack, the hero tries to attack him, only for Homeless Emperor to blast his head and later his heart, and block a point-blank shot from Zombieman's hidden gun. While Zombieman regenerates, he lures Homeless Emperor into a conversation to buy time. As Homeless Emperor relates his backstory and how he got his power from "God", he prepares to finish Zombieman.
Elsewhere, as Sweet Mask wonders about how the battle consumes time on his schedule, he finds the devices which nurture the Monster Cells and destroys them. He is later ambushed by Fuhrer Ugly. As the monster states he hates heroes like him, Sweet Mask recognizes the monster as an Ugmon and states he cannot win against him.
Pig God encounters Gums and they battle. Iaian, Okamaitachi and Bushidrill hunt down a monster that escaped them and find it in a vault room. In the vault room, the monster knocks down a fish tank containing Evil Natural Water and the Mad Doctor Fish residing with it. The Mad Doctor Fish kill the runaway monster and attack the heroes. The heroes prepare to engage with the monsters. Puri-Puri Prisoner is swimming through the concrete of the Monster Association Base.  He manages to discover a chained up Garou and, when unwittingly trying to free him, is brutally attacked by the Hero Hunter.
On the surface of the Monster Association base, the support heroes are handling the remaining monsters and patching up the entrances to the Monster Association HQ. Nyan suddenly appears and One Shotter shoots the monster. Nyan flicks the bullet back at One Shotter, taking him out. Sekingar orders Food Battler Futoshi to take Waganma back to the Hero Association HQ while the rest of the heroes fight Nyan. Nyan cuts through them and takes Waganma. The monster is forced to let go of the hostage when he is attacked by Drive Knight. Drive Knight and Nyan begin to battle each other. In the end, Nyan is defeated by Drive Knight.
*There are also numerous unnamed members.
Webcomic and Manga Differences
- The Monster Association is expanded upon in the manga, having more members and having them come out to the public. In the original webcomic, it only had 17 members: Black Sperm, Bug God, Devil Long Hair, Evil Natural Water, Fuhrer Ugly, Gale and Hellfire, Goddess Glasses, Gums, Homeless Emperor, Overgrown Rover, Phoenix Man, Psykos, Pureblood, Royal Ripper, Senior Centipede and The Great Food Tub. In the manga, however, they have around 500 members.
- All of the members in the webcomic are Demon or above.
- Monster Cells were introduced in the manga, and never existed in the webcomic.
- In the manga-only, The Organization sent G5 to assist the Monster Association, tying the two factions together.
- The leader is Psykos, as Orochi does not exist in the webcomic.
- The idea of the Monster Association headquarters being previously the home of the Subterranean People was a suggestion by a fan on stream.
- In both the webcomic and the manga, the Monster Association mirrors the Hero Association.
- In the webcomic, the Monster Association has 17 members, which is the number of S-Class heroes in the Hero Association.
- In the manga, the Monster Association has around 500 members, which is roughly the number of pro heroes in the Hero Association.
- Monsters who are not a part of the Monster Association are sometimes referred to as freelance monsters by members of the association.