Mysterious Beings/Monster Association

The Monster Association (怪人協会, Kaijin Kyōkai) is a Mysterious Beings group led by Gyoro Gyoro and Monster King Orochi. Not much is known about it, except that they are the antithesis of the Hero Association.

History
The Monster Association is an unknown group first introduced to Garou by two members of the said organization. They tried to invite him but are turned down, despite being unlikely friendly.

Its leader Orochi defeated the martial artist Gouketsu and recruited him as a member.

The ninjas, Gale Wind and Hellfire Flame, join to kill the hero Flashy Flash.

2 years ago, Elder Centipede joined to avenge his defeat against Blast.

There are five primary objectives of the organization: Any monsters who don't join the Association are considered to be free monsters. It is shown that all monsters in the Association only join force to destroy the Hero Association and will not submit to Orochi as their leader in which some of them like Black Sperm or Gale Wind and Hellfire Flame will try to challenge Orochi's authority over all Mysterious Beings after the Hero Association falls.
 * 1) Strike fear to the human population through monster's rampage.
 * 2) Kidnapping important hostages that are vital to the operation of the Hero Association.
 * 3) Transform several powerful humans to boost military strength of the organization.
 * 4) Destroy the main force of the Hero Association, which is the S-class heroes, to weaken the Hero Association so they can attack the Hero Association HQ.
 * 5) Build the world where monsters reign over humanity.

The monster members also very cocky to think that they are stronger than Hero Association without taking note of the power of S-class heroes in which Pyskos has to remind them that their arrogance will be their downfall if they constantly underestimate the S-class heroes.

The Monster Association consists of 500 monsters overall. There are around 30 known demon-level monsters, although many of them were killed in the raids against the cities.

Rumored Monster Arc
Rumors have been spreading that strong monsters have been meeting up in Z-City's ghost town and that Humans and even some Mysterious Beings were avoiding the location.

Monster Raid Arc
In S-City, Venus Mantrap and Junior Centipede were first members to attacked a sushi restaurant where the Hero Associations sponsor Narinki and his son Waganma were at. Metal Bat, who was guarding them, easily defeated both monsters, however Rafflesidon and Senior Centipede came erupted from the ground. Both Narinki and Waganma were knocked out by Rafflesidon's gas and prepare to fight Metal Bat. Metal Bat fights the monsters, but was slightly sedated by the gases the monster released. Metal Bat, however managed to break out of its control by smashing his head and killed both monsters with one strike each.

Moments later, Elder Centipede, a Dragon-level threat, emerged through the ground. Elder Centipede is after Narinki and avoids Metal Bat's attacks. As Elder Centipede was about to catch the sponsor, he was struck by Metal Bat and tries to shake Metal Bat off by maneuvering around the city. Elder Centipede managed to launch Metal Bat far away and continues to rampage through the city. Just as Elder Centipede cornered the heroes Pineapple and Mohican, who were carrying the Narinki and Waganma, it was attacked by Metal Knight missiles. Phoenix Man, Sludge Jellyfish and Rhino Wrestler then corner the heroes and demand the sponsor. After Garou finished 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 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 monster say they'll overlook Garou's rude behavior this time and promises that they will meet again, if Garou continues to hero hunt. Meanwhile Rhino Wrestler fights and defeats Pineapple and Mohican and takes Waganma as hostage. The monsters then retreat after explaining how fighting Metal Knight would only be in vain for them. Phoenix Man instructed a retreat to Elder Centipede, and him along with Rhino Wrestler were taken by the large Monster and burrowed underground, but Metal Knight managed to stick to the monster.

The Monster Association has attacked many cities simultaneously:
 * W-City: Heavy Kong has been struggling against Marshall Gorilla.
 * D-City: Lightning Genji fight Maiko Plasma and Electric Catfish Man.
 * I-City: Hundred-Eyes Octopus causes havoc
 * Y-City: Eyesight causes havoc.
 * V-City: Pureblood causes havoc.
 * Unknown city: Do-S and Free Hugger causes havoc.

Fist Fight Djinn is fighting the A-Class hero Smile Man, when Bang arrived at the scene. Fist Fight Djinn attempt to attack Bang, but was mercilessly killed by the S-Class hero. Meanwhile, Do-S managed to defeat many heroes and was then confronted by Fubuki and her Blizzard Group. Marshal Gorilla managed to defeat Heavy Kong and tells the civilians to spread the word about him. Maiko Plasma and Electric Catfish Man are still fighting Lightning Genji and hospitalize him. Hundred-Eyes Octopus kept devouring building and easily managed to defeat Death Gatling, Butterfly DX and Bone. Eysight managed to poison Ecolo G and other heroes and is fighting Studless. Pureblood managed to kill many civilians.

Many monster that appeared in C-City were defeated by Genos and Face Ripper later encountered the hero. Face Ripper managed to damage Genos, but was caught off guard and killed. In the same city, Awakened Cockroach defeated many heroes and meets Genos in an alley. On the TV, it was reported that Lightning Genji was defeated by the monsters, while Awakened Cockroach managed to have an upper-hand in the fight against Genos, but was caught in an adhesive trap, but managed to escape before being struck by Genos' blast.

Do-S managed to transform all of the Blizzard Group members into her love slave. While Fubuki was preoccupied by the love slaves, Do-S managed 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 reveled her plan of using Fubuki to lure Tatsumaki out. However, Do-S was sent flying and attacked by Fubuki. Angered she decided to kill Fubuki. When the area began to rumble, a monster falls in front of Do-S ans warns her about Tatsumaki. Frightened, Do-S orders her love slaves to distract Tatsumaki, while she escapes.

Super Fight Arc
The Monster Association still continues their attack, Sweet Mask's concert was interrupted, the hospital where Mumen Rider, Tanktop Master and the Tank Topper Army were staying was attacked and Nyan broke into Smelly Lid Prison in search of Puri-Puri-Prisoner. At their hideout Gyoro Gyoro and Monster King Orochi were discuss their invasion and Gouketsu whereabouts. They managed to defeat S-Class heroes like Genos and Metal Knight, whose robot hanging was severely damaged and captured at Monster Association's Headquarter, and its broken pieces are hung in chains.

Despite their succession, many monsters were killed by S-Class heroes. Drive Knight easily killed some members and interrogated the last living one about the invasion. Child Emperor used Underdog Man No. 22 to fight Eyesight, but the monsters was later eaten by Pig God. Hundred-Eyes Octopus's destruction forced to assemble an A-Class heroes team, but was later blinded by Flashy Flash and crushed into a ball by Tatsumaki. Gyoro Gyoro worries slightly about the S-Class's poweres, 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, one of the Council members Haragiri, transformed into a Mysterious Being by injecting himself with Monster Cells and orders the others to do the same. Atomic Samurai rejected the offer and killed Haragiri. Through his robot Metal Knight was able to spy 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 fight the monsters and protect the patients and staff members. Mumen Rider struggles against Gyoffrey, but was saved by Tanktop Master.

Gouketsu and The Three Crows interrupted the closing ceremony of the Super Fight Martial Arts Tournament in an attempt to transform all of the Martial Artist into monsters. Gouketsu brought one Monster Cell for each martial artist. Rosie, Choze, Benpatsu, Hamukichi and Volten all joined the Monster Association, though Rosie was quickly killed by Choze. While the other martial artist were frightened, Suiryu decided to rebel against them. After Suriyu managed to defeat all of the martial artist, Gouketsu was impressed and offered Suiryu and executive position in the association. He was rejected and Gouketsu struck 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, Bakuzan instead ate several monster cells and collapsed on the floor, seemingly dead. Gouketsu then ordered The Three Crows to finish off Suiryu, but the Crows were later killed by Suiryu in a team effort with Lightning Max and Sneck. After the Crows were killed, Gouketsu was challenged again by Sneck and Lightning Max. He easily defeated them and when Suiryu tried to leave the stadium, Bakuzan woke up from his transormation and block his entrance. Bakuzan then continuesly strikes Suiryu on the ground.

Gyoro Gyoro uses its spy drones to watch the chaos in the city and confirmed that their first objective was successful, as they also captured Waganma, the son of a Hero Associations sponsor. Phoenix Man noted they lost many monsters and asked for reinforcements, but the request was denied. The monster noted 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. Bakuzan refuses and fight Gouketsu. Gouketsu then warned Bakuzan about his attitude and informed him of Orochi, the strongest being on Earth. He gives Bakuzan a firm warning as he leaves the stadium for Z-City, the meeting point of the Monster Association.

As Gouketsu was heading towards the hideout, Saitama came and they fought off-screen, with Gouketsu's decapitated head comes flying into the stadium and lands in front of Suiryu. Free Hugger managed to slightly injure Puri-puri Prisoner, but the monster was squeezed to death by the hero.

Monster Association Arc
Elsewhere, Gale Wind and Hellfire Flame attack Speed-o'-Sound Sonic who was training in the forest. As they were discussing, the ninjas reveal they have joined the Monster Organization and with their advanced training managed to obtain their human form. Gale Wind and Hellfire Flame reveal they joined the Association to rule the world, more specifically to kill Flashy Flash and ask Sonic to join them. Sonic initially refuses, but when Gale Wind demonstrates his speed after his transformation, Sonic was persuaded and took a Monster Cell. Later Sonic cooks the Monster Cell into a gourmet meal, however didn't transform into a monster instead had an upset stomach afterwards.

In W-City, Marshall Gorilla still wonders around the streets, feeling unsatisfied by his threat level being Tiger and wants the Hero Association to change it to Demon. Zombieman was following the monster and wants to gather as much information about the organizations and its hideout. Marshall Gorilla then spots someone on the streets and was surprised to see Armored Gorilla, who was shopping for groceries. Marshall Gorilla felt angry that Armored Gorilla "sided" with the humans and attack 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 can declare war on them. He kills one of the association members before being killed by Superalloy Darkshine. In Z-City, Gyoro Gyoro gives a speech to the monsters and Orochi decide to eat Awakened Cockroach for failing to kill Genos. Orochi wanted to kill Do-S as well, but decided not to, since Gyoro Gyoro stated her powers are useful. When they noticed Gouketsu's absence, Gyoro Gyoro used one of its drones, only to find Goukestsu's headless corpse.

When Garou is being attacked by Genos, Bang and Bomb, some of it's members arrive to save him, but are swiftly beaten.

Elder Centipede then attacks and overwhelms Bang, Bomb and Genos, before being killed by Saitama. Meanwhile, Phoenix Man escapes with Garou.

After Saitama defeats Garou, the organization collapses. Psykos is taken prisoner. Homeless Emperor is killed by "God". Pig God digests Evil Natural Water. So far, Black Sperm and Black Rover are the only surviving members of the organization but they are now living as Saitama's pets.

Members
Dragons= Demons= Tigers= Unknown=

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 web-comic, 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 over 500 members.
 * In the web-comic, they also didn't have tiger-level threats.
 * Monster Cells were introduced in the manga, but never existed in the web-comic.
 * They are also apparently affiliated with The Organization in the manga.