Hosu Incident

The Hosu Incident (保須事件) is a villain attack on Hosu City conducted by the Hero Killer: Stain and the League of Villains.

Prologue
After brutally murdering seventeen heroes and injuring twenty-three beyond recovery, Hero Killer: Stain becomes a scourge on the entire hero community. The Turbo Hero: Ingenium and his squad search for Stain in Hosu. Ingenium eventually finds Stain in an alleyway and confronts him. The Hero Killer manages to grievously injure Ingenium and leaves him for dead.

The news of the attack draws the League of Villains to Stain. They approach him on the roof of a building and invite him into their hideout. Tomura Shigaraki asks Stain to join their party. Stain asks for Tomura's motivations and is deeply disappointed with the answer.

The villains come to blows, Stain paralyzes Kurogiri, then stabs Tomura and pins him to the ground. Stain tells Tomura that he's weak because he lacks conviction. However, an enraged Tomura destroys Stain's knife and forces him back. He tells the Hero Killer that he desires to destroy society because it praises All Might. Intrigued, Stain realizes that they both agree to destroy the present. Tomura demands he leave and Stain explains he attacked him to test his motives. Kurogiri is freed and he agrees to return Stain to Hosu.

Tomura and Kurogiri follow Stain to Hosu. Tomura asks Stain what he plans to do there. Stain explains he must spill blood in order to reform it. Kurogiri commends his efforts. Stain goes on to say that hero is a title for great champions, not people who desire glory and payment for their heroic deeds. Then he jumps into the city and continues his work. Tomura claims that Stain is wasting his time, but Kurogiri argues that Stain has positively affected all the cities he's appeared in.

Irritated, Tomura mocks Stain and claims he can never work with the Hero Killer. He requests three Nomus from All For One and has Kurogiri warp them to Hosu City.

Incident
The Nomu attack the city by destroying buildings and assaulting civilians indiscriminately. Heroes mobilize to the site of the chaos. One Nomu attacks a nearby train, which happens to harbor Izuku Midoriya and Gran Torino. Gran Torino tackles the Nomu off the train and saves the Pro-Hero it was attacking. Izuku exits the train to search for Gran Torino. Manual takes Tenya Ida towards the center of the incident, but the young man veers off the path to confront Stain.

The Hero Killer finds his next target in the Pro-Hero Native. He paralyzes Native and prepares to kill him. Tenya intervenes and rushes Stain, but the villain counterattacks and slashes Tenya's helmet off. He tells the child to run away, but Tenya declares that he will avenge his brother. Stain is displeased with Tenya's search for vengeance and decides to make him a target of his purge.

Izuku uses One For All: Full Cowl to jump through rooftops and search the city for Gran Torino. The Nomu Gran Torino's fighting recovers and attacks nearby civilians. Gran Torino stops him with the help of Endeavor. The citizens of the city frantically evacuate while Izuku continues running through the streets in search of Gran Torino. He realizes the entire city could be in trouble if this Nomu is like the one from the U.S.J. Incident.

Izuku notices Manual's voice calling for Tenya and happens upon a scene where two more Nomu are dominating Pro-Heroes in a fight. Manual helps put out the fires caused by their destruction. Izuku overhears him talking about how Tenya went off on his own. One of the heroes tells Izuku to evacuate and then he realizes that Tenya likely encountered Stain. Izuku leaves while the Nomu continues to defeat the Pro-Heroes.

Endeavor and Shoto Todoroki notice the havoc arising in Hosu and rush toward the scene of the crime. Shoto leaves after receiving a location from Izuku and asks Endeavor to send any Heroes he comes across to that address.

Gran Torino fights the Nomu to stop it from attacking random civilians. Endeavor arrives on the scene and aids Gran Torino in defeating the monster. The Flame Hero asks Gran Torino to investigate the location given to him by Shoto. Endeavor goes alone to help the other heroes struggling against the remaining Nomu. Endeavor kills one Nomu by incinerating its head and injures the other to save an unconscious hero from its clutches.

Izuku surmises that the League of Villains and Stain are connected. He searches the back alleys of Hosu, looking for Stain far away from the chaos because Stain normally operates in the shadows. Izuku successfully finds Tenya and the Hero Killer just in time to save the former from death. Izuku fights Stain, but is ultimately defeated and paralyzed by the villain's Quirk.

Stain is impressed with Izuku's heroic nature and allows him to live. The Hero Killer tries to kill Tenya, but he's stopped when Shoto arrives and attacks using his fire. Shoto and Stain begin to fight and the villain starts to overwhelm the U.A. prodigy. Izuku regains his movement and they both work together to fight the Hero Killer. Even their combined efforts aren't enough to stop the villain, forcing Tenya to rethink his motives and join the fight.

The trio works together to finally defeat Stain with an epic combo attack. They restrain the Hero Killer and bring him back towards the street. Tenya apologizes for his selfish behavior and cries when he asks for his two friends' forgiveness. They are all eventually found by Gran Torino and other Pro-Heroes. The last remaining Nomu suddenly arrives and takes Izuku. Stain regains consciousness and kills the Nomu to rescue Izuku.

Endeavor arrives and recognizes the Hero Killer. Stain becomes irate at the sight of Endeavor and unleashes his bloodlust. He tells the heroes they are all shams and that he will reform society. His words strike fear into everyone present. The conflict only ends when Stain falls unconscious because one of his broken ribs pierces his lung.

Aftermath
Izuku, Shoto, and Tenya are admitted to the Hospital where they are confronted by Gran Torino, Manual, and the Police Chief Kenji Tsuragamae. Shoto and the Police Chief get into an argument about the students illegally using their Quirks to combat Stain. However, Chief Tsuragamae decides to pardon the boys of their crimes because they helped stop the Hero Killer's crime wave, although this is done under the condition that the boys cannot publicly be credited or receive the glory and fame as the ones who defeated Stain, the credit instead going to Endeavor.

Following the defeat and arrest of the Hero Killer, Tomura and Kurogiri are caught on camera by the media. They are all assumed to be associated with one another and the blame for the incident is pinned on the League of Villains. Stain becomes a media icon and details about his life are released on the internet. His motives, origin, and ideology all become widely known through a video becoming widely spread across the internet. The charisma displayed by Stain and his righteous ideology help inspire a new wave of villains that all move toward the League of Villains.