The scene picks up with the five students having not only to deal with a serial killer, but now a doomsday machine built to kill people with quirks.

Y/N: So, the Sentinels are still function after all these years. Whoever decided to reactive them must be desperate to want the heroes dead. 

Sentinel: QUIRKS DETECTED. TERMINATE ON SIGHT. 

Its arm turns into a mini-gun and shoots at the group. Everyone manages to take cover from the shots with Todoroki making an ice wall to try and reduce the shots. Stain growls in anger by the robot.

Stain: That abomination is a threat to everything if it continues to live. So, you all can choose to either fight it or me. Your choice.

Lightning: As much as I hate to say it, he's right. That thing's too dangerous to leave unchecked.

Shoto: But what about the Hero Killer? This could be our only chance to catch him, and we still don't know where the pros are.

More shots are fired until Robbie whistles. All the sudden, his dodge charger zooms right through the Sentinel, making it loose balance. It falls down and stops shooting. With the distraction, Stain goes in for the kill, but is suddenly hit with a front kick by Iida.

Deku: Iida!

Shoto: You're free, too. Guess his quirk isn't as great as I thought it was.

Lightning: Good to have you back, Ingenium.

Ingenium: Farron... Reyes... Todoroki... Midoriya... This has nothing to do with you. I apologize.

Lightning: Fuck's sake, don't start this again.

Ingenium: I'm okay. And I won't let the four of you shed any more blood for me!

Stain: It's no use trying to pretend you're a hero now. A person's true nature doesn't change in just a few minutes. You'll never be anything but a fraud who prioritizes his own desires. You're the sickness that's infected this society and ruined the name "hero." Someone must teach you a lesson.

Ghost Rider: Pot calling kettle black. You claim to be fixing society by killing false heroes, but the truth is you're a psychopath who does it for the thrill of it. A fundamental lunatic that will remain as a poison to the people until he himself is removed. You're not worthy to be killed by All Might.

Stain was now furious by that and rushes in but Robbie and Todoroki blast fire at him. As they do, the robot begins to get up.

Pro Hero: That thing's getting up again!

Deku: What should we do?

Ingenium: The best we can is divide and conquer. Farron, Reyes, you two are our strongest heroes right here. Try and hold off the Sentinel until the pros arrive. The rest of us will handle Stain.

Lightning: Are you sure?

Ingenium: I am. Don't worry. My mind is clear now. Both of you please try to hold him off as long as you can.

They saw his face, and saw determination in Iida. Not for trying to kill Stain, but to help save his friends. Agreeing with him, Arc and Robbie head out and see the Sentinel fully back up.

Sentinel: QUIRKS DETECTED. TERMINATE.

It begins to fire its weapons again and the two students dodge the shots while rushing in.  

Lightning: If I remember, these things were weakest inside. If we can get into it, we can bust it apart.

Ghost Rider: They're also not very good at taking big attacks. Hit it with as much force as possible.

The machine fires at them as they run in to deliver powerful strikes, making it take some hits. It proved difficult as the armor was very durable and it had fast reflects. Robbie slashes it multiple times and Arc runs around it to deliver speed punches.

Arcturus: (thoughts) This isn't working. For something so old, this thing's durability is high. If I wanna get through, I need a good clear shot!

Lightning: Rider, just to hold it in place. I'm gonna try something.

Ghost Rider: What?

Not specifying more on it, Arc runs off at a distance. He stops after a bit and turns back around.

Arcturus: Alright. Feel the air... The rush of adrenaline... And just go for it.

His eyes spark up and he begins running. Faster and faster he goes to the point where he's building up pressure. The Sentinel sees him coming towards him and starts firing as Robbie throws more chains to hold it down. Arc dodges the shots and gets closer.

Arcturus: (thoughts) Focus! Focus! Focus!

Jumping up, Arc gets ready to throw a punch. And then, at the last second of impact, he begins vibrating his whole body to the point where he passes right through the torso of the robot. 

*BOOM!!*

In a loud sonic boom, the entire back of the Sentinel explodes with Arc shooting out!

Lightning: SUPER SONIC PUNCH!!

Arcturus: (thoughts) Yes! I did it! I phased right through that thing and damaged the core of it!

Acting fast again, Arc skids himself to stop to the point where he nearly breaks the pavement. For that moment, Arc relaxes, until he sees the machine still active.

Arcturus: (thoughts, gasps and eyes widen) No... I blew out that thing's back! How the hell is it still active?!

Ghost Rider: Arc!

Sentinel: QUIRKS DETEC—

It's interrupted by a bunch of energy spears shooting right at it. The boys look over and see Shadow not too far from them.

Lightning: Shadow!

Shadow: You two hang back. I'll finish the rest.

Now the Sentinel was more damaged as it tries to shoot. However, Shadow rushes in, and using a technique similar to Arc's spin dash, he jumps around the buildings nearby and damages the robot with each strike. In just a short amount of time, Shadow destroys the Sentinel completely.

Ghost Rider: Impressive.

Lightning: You can say that again. Thank you, sir—

He's stopped by the furious glare Shadow sent at him. Admittedly, both Arc and Robbie couldn't help but freeze in fear by it.

Arcturus: (thoughts) Oh crap. He's probably mad because Izuku and I didn't tell him where we were going.

Shadow: When I said we were going to fight villains, I didn't mean go toe-to-toe with a murder weapon!

https://youtu.be/S3wsCRJVUyg

At the his fight, Shadow was finishing up killing the Nomu as Endeavor joins in to help. Once it was finished, they inspected the damage.

Shadow: I've seen many freaks in my time, but this takes the cake.

Endeavor: No kidding. Why are you here?

Shadow: Could ask you the same. I'm in the middle of an internship for— (notices something) Wait a second, did you see two boys nearby? One's green and the other's blue.

Endeavor: No.

Shadow's communicator goes off and he sees it's the operative.

G.U.N. Operative: Shadow, come in! We're getting sightings of a Type-3 Sentinel on Echo Street in District 4! It also looks like seven people are there, 5 of them appear to be teenagers! 

Endeavor: That's the address Shoto was heading to!

Shadow: (shocked) What?! (to his comlink) I'm heading there now!

Without even waiting for Endeavor, Shadow runs off to the destination.

Shadow: (thoughts) No doubt Farron and Midoriya went there, too. And Shoto's there now. If anything happens to him, Fuyumi's gonna kill me.

(now) 

Shadow: This is beyond reckless from all of you. Have anything to say for yourself?

Arcturus: (takes his mask off) I'm sorry for running off without telling you, but I have a good reason.

Shadow: Which is?

Izuku: (off screen) Arc!

They look over and see Izuku, the pro, Todoroki, Iida, and an unconscious Stain tied up approaching them. Seeing Stain, Rider gets ready to head over to him but Shadow stops him. 

Shadow: Stand down.

Ghost Rider: He must face judgement.

Shadow: He will, but not now. It's over.

Reluctantly, Robbie deactivates his quirk and his human flesh returns. As Izuku and Shoto lifted Stain up, he took a glance up and saw what looked to him a glimpse of someone. But when he blinked, they were gone.

Shoto: Midoryia, are you okay?

Izuku: H-Huh? Yeah, come on. Let's get out of here.

The other pros arrive there and one of them call in an ambulance. As they were checking their injuries, Iida approaches his classmates and bows to them while having tears in his eyes.

Iida: You guys... You all were hurt because of me. I'm truly sorry. I was just so angry. I couldn't...see anything else.

Izuku: I'm sorry, too, Iida. You were going through so much by yourself. But I couldn't tell if anything was wrong.

Arcturus: Yeah. I thought something was bothering you, but failed to help. Guess we both screwed up in that. 

Shoto: Hey, pull yourself together. You're the class rep.

Iida: (wipes his tears away) Right.

Robbie: But next time something like this happens, just talk with us. Trust me. From my experience, this kind of shit doesn't go good when you go for vengeance.

Suddenly, a damaged Winged Nomu comes in and grabs Izuku, shocking everyone despite it bleeding.

Arcturus: Izuku!

Shadow: Drop him!

Y/N who was watching from the rooftop, instinctively prepared to do something, but before he could, Stain acted first when he licks the Nomu's blood, making it freeze up, he cuts his bindings off, runs in, jumps up, grabs Izuku free and kills the Nomu. Everyone is shocked by the action and even on edge.

Stain: Everything I do... 

is to create a stronger society.

Shadow: Step away from the boy, Stain! Now!

He and Arc get charged up to run in as Endeavor does the same.

Gran Toorino: Wait, you three!

Stain's mask falls off and turns back at the heroes. When he does, everyone suddenly felt an overwhelming sense of fear and intimidation from his gaze.

Stain: Endeavor... You false hero. I'll make this right. These streets must run with the blood of hypocrites. Hero! I will reclaim that word!

As soon as Stain would look at everyone, they were all paralyzed in fear and couldn't move, including even Shadow, Endeavor and Gran Torino from the malicious aura.

Stain: COME ON!! JUST TRY AND STOP ME, YOU FAKES!!! There is only one man I'll let kill me! He is a true hero! ALL MIGHT IS WORTHY!!!!!!!!!!!!!!!

The bloodlust coming from Stain was felt by every hero there. Pro's and students all felt their bodies freeze from the pressure of Stain's words. Even the mighty Endeavor stood in fear of the man in front of him. And so did Y/N. He felt afraid of Stain and his words. It was then that Y/N realized the commitment of Stain's mission.

The feeling keeps up until it suddenly stops and Stain just stands still. Or more like he was unconscious while standing. 

No one spoke and some couldn't help but fall on the ground. Some like Izuku were actually still petrified with fear by what they had seen.

Arcturus: (narrator) It wasn't until later that I heard what happened. A quirk didn't stop the Hero Killer. One of his broken ribs pierced his lung. He hadn't managed to lick anyone's blood. But still, in that moment, none of us could bring ourselves to stand against him. Only the Hero Killer had any fight left.

Y/N fled from the roof and seeped into the shadows as he fled the scene. But he didn't fully escape the area, he was just in hiding, waiting for something or more importantly someone.

Y/N: Beros. Leviathan. I need you two for something. 

*???*

In an unknown location, the man from earlier was reviewing some of the footage from the Sentinel, including the aftermath with Stain.

Man: Even heroes like Endeavor couldn't defeat him, but mere children did. It's honestly embarrassing for a man like him

 From a computer monitor, All For One speaks. 

All For One: I could say the same for the Sentinel. Even with the minor updates from the original design, it still proved worthless.

Man: My goal was not to kill those students. Even if that would be a bonus from the attack. I actually collected a much necessary data as I required.

On various machines, it looked as though images of Arc, Izuku, and all the people at the scene were scanned.

Sentinel: ALL LIFE DATA SUCCESSFULLY COPIED.

Man: I'll require more quirks to analyze, but once I'm done, I'll have the means to bring this world into the perfect image. 

All For One: You never cease to amaze me, old friend. At this rate, the heroes will be gone, and a new world will come to pass.

Man: Yes. And she will be by my side once again.

(back at Hosu) 

In a truck driving down to Tartarus, some police were bringing a defeated Stain to the super criminal prison. As they were driving, Leviathan suddenly came and pushed the truck with his fist. 

Crashing into a road divider, the police were knocked out. Suddenly the door to the back was ripped open and it was revealed to be Y/N with Beros and Leviathan.

Stain: (weakly) Y-You....

Y/N helped Stain out of his cuffs and made sure to handle him with care, seeing as he was badly injured due to one of his ribs being in his lungs.

Stain: (weakly) Why....are you....here....?

Y/N: You asked me what my mission was remember?

Stain looks at Y/N weakly as he listens to what the boy has to say.

Y/N: My mission is a very important one. But there's some things I need to do before I can make it happen.

Stain: (weakly) What....does that....have to do.....with me...?

Y/N: I'm sure you don't want to rot in a cell at Tartarus, so I'm offering you a choice. You help me, and I help you.

Y/N extends his hand to Stain waiting for him to come to a decision. In a few moments, Stain grabs hold of Y/N's hand, causing him to smile a bit.

Y/N: Let's keep this alliance between us. I don't want Shigaraki poking his nose where it doesn't belong.

He takes Stain and puts his arm over his shoulder and takes him somewhere to get some medical assistance with Beros and Leviathan following him as the truck begins to catch fire.

Chapter 9 ended. 

And that's the end of the Hero Killer Arc. I hope you guys like it. Just a few more, and season 2 is done. Let me know what you guys think at the comments below, peace out and enjoy!