Play Inception Time by Hans Zimmer

"If you're wondering, I'm the one who made the letter," (Y/N) explains further, "Tenko started going crazy when my dad first brought him back. I didn't like it, but I stayed because I still loved my father. Eventually, Tenko and I called each other siblings. We grew close, but I couldn't take seeing my brother and father so crazy. After finding out that my father was considered a villain, I wrote a note out as best I could while copying his handwriting. I teleported to the front of UA High and laid in that basket. I could talk to you all with my mind then too, but I didn't know if you were trustworthy."

"We can deal with this later." Iida says in his mind.

"We have to save Bakugo, though," They tell them, "He's surrounded by the villains and I have an idea."

(Y/N) and Midoryia work together for a plan, eventually coming up with one that could work. Iida and Midoryia use their quirks to get Kirishima and themselves moving. Todoroki and (Y/N) create a ramp together that throws the three boys towards the wall. Kirishima uses his hardening to break through it. As they fly over the battlefield, Kirishima calls out to Bakugo. The blonde boy complies and flies over with his quirk.

The League of Villains attempt to grab Bakugo again by using Magne's quirk. (Y/N) notices this and quickly teleports up into the air. They create a portal around all the villains and teleports them back towards where they originally came from. (Y/N) teleports back to Todoroki and their escape mission is over.

After the group lands, (Y/N) has seemed to disappear.

Back with All Might, the other villains were forced into Kurogiri's portal, so it is only the number one hero and Gran Torino who can fight. The other heroes from earlier are still out cold. (Y/N) appears as All for One is just talking about the past. They're directly in the middle of the two. They take off their disguise.

"(Y/N)?" All Might and All for One say in unison.

The child ignores them, charging up their version of All for One's quirk, as well as Todoroki's. Their left and right side get covered in ice and flames.

"(Y/N).." All for One says, "You've finally learned to control that power of yours. Why'd you run off? I could have helped you!"

"That's just it," They respond, "I never wanted to be taught by you! You're a villain! You're horrible, dad!"

All Might's face contorts, and his eyes darken. Now, he looks more like a villain.

"I told you who Nana Shimura is remember, (Y/N)?" He asks, gesturing to All Might, "This is her successor."

(Y/N)'s expression still doesn't change. They glare at their father, and teleports beside him. They light their father on fire and attempts to put him into a 'frozen-shock' by also covering him in ice at the same time. He chuckles and uses forced quirk activation on them to teleports them away. They land in the arms of Midoriya as he and Bakugo watch the fight.

"NO!" They yell out, trying to get out of the green-haired boy's grasp.

"(Y/N), stop it," He hisses, "You can't go fight him. All Might will handle him. You'll be able to see him again. Just trust the heroes and let them do the hard work."

The child finally complies and watches their real father fight the number one hero. All for One fires a large blast that All Might must dodge to keep most of his strength. He notices a civilian nearby, so he stays put and blocks the attack as much as possible. (Y/N) reads their minds, trying to understand their conversation through it. They understands that their father has told All Might the truth. Specifically that (Y/N) is actually All for One's daughter and that they wrote the note themselves. He also mentions that Shigaraki is actually Nana Shimura's grandson. All Might nearly loses control, but other heroes wake up and help the nearby civilians leave. He can almost hear the world cheering. (Y/N) sends a message to the number one hero, one that he will never forget hearing from the baby. But for once, they say it outloud...to the entire world. Their thoughts that they meant to send to All Might alone, instead, heads to the entire world.

"COME ON ALL MIGHT! TAKE DOWN MY FATHER AND END THIS FOR GOOD!"

Chills are sent down the spines of the entire world as (Y/N)'s quirk affects them. Anyone who wasn't watching the news feed quickly turns on the tv to find out what's going on. All Might punches All for One in the face after dodging an attack, thanks to (Y/N)'s words and the villain falls to the ground.

After he raises his fist and the search for fallen civilians begins, he turns to the news camera and points, "Now," He says, "It's your turn."

Midoryia gets distracted and forgets that (Y/N) can hear his thoughts. They know the truth, that Midoryia is the whole reason that All Might almost lost a fight against their father.

"Deku.." They mutters, "It's...your fault?"

The green-haired boy's eyes widen, and he doesn't realize that he's just started World War III...against a baby with a thousand quirks.