Monster Association Arc

The fifteenth arc of the series and the sixth of the Human Monster Saga, this is by far the story's largest and longest campaign, spanning roughly twenty volumes. It stages an all-out war after the Monster Association kidnaps a sponsor's child and declares open hostility, sending the Hero Association's full S-Class into a sprawling underground fortress while Garou's descent toward becoming the ultimate monster reaches its climax.

After the ninja pair Gale Wind and Hellfire Flame lure Speed-o'-Sound Sonic into swallowing a monster cell, the Monster Association formally declares war, murdering a Hero Association executive through a parasite before Superalloy Darkshine intervenes. Garou, still recovering, is ambushed by Death Gatling's squad of heroes and beats them all, only to be cornered by Genos, then Bang and Bomb. Phoenix Man and Elder Centipede rescue him, and the towering centipede is annihilated when Saitama, hidden behind King, delivers a Serious Punch.

Dragged into the enemy's base, Garou is pitted against savage cadres and slowly broken down, first by Royal Ripper and Bug God, then by the overwhelming Overgrown Rover, before the Monster King Orochi crushes him under Gyoro Gyoro's guidance. Meanwhile the Hero Association mounts a rescue for the captive boy Waganma, and Child Emperor quietly strikes Genos and Bang from the roster before the S-Class pours into the fortress buried beneath Z-City. There they meet an army of over a hundred and fifty monsters led by Evil Eye, whom Tatsumaki obliterates.

The raid splinters into vicious duels across the base. Flashy Flash slays the two ninja monsters, Child Emperor destroys Phoenix Man and frees Waganma, Zombieman outlasts the vampire Pureblood, and Sweet Mask butchers Do-S and her enslaved mercenaries. Others fare worse: Atomic Samurai is buried by the endlessly dividing Black Sperm, Zombieman is battered by the god-touched Homeless Emperor, and Sweet Mask freezes at the sight of the hideous Fuhrer Ugly. Deep below, Saitama wanders the tunnels almost by accident and ends Orochi with what he calls an ordinary punch, then crushes Gyoro Gyoro, whose true self proves to be Psykos, a former friend of Fubuki.

Cornered, Psykos drinks a mysterious elixir and fuses with Orochi's remnants, gaining power she attributes to a glimpse of a godlike entity; her battle with Tatsumaki tears the earth's crust skyward and spawns a colossal tsunami. Garou, meanwhile, terrorizes Superalloy Darkshine into paralyzing fear and completes his monstrous evolution. When the awakened Hero Hunter threatens the child he once protected, Saitama steps in, shrugging off every attack and flipping the entire fortress twice over to fight seriously.

The war ends in a Hero Association victory as the Monster Association collapses. Saitama shatters Garou's remaining monster mutation and argues that the Hero Hunter secretly longed to become a hero the entire time, having cast himself as absolute evil only to unite the world against a single villain. Though the heroes move to finish Garou off, Saitama stalls them and the boy Tareo intervenes, and a newly hopeful Garou escapes. The conflict also hints at a larger menace, as the top-ranked hero Blast is revealed to stand against the same mysterious 'God' that empowers the Association's strongest foes.