Snake and the All For 1
Medusa comes to the UA Gang and All Might's aid when the League of Villains attacks the Unforeseen Simulation Joint.
Disclaimers: I own nothing
Roof Tops Unknown Location
Medusa is checking her old sketch book during her old times as Izuku Midoriya, in past as him, he always sketches out all kinds of Heroes that he witness in action against a Villain and write their Quirk Powers in each of the pages of the Hero he sketches. Lastly she finally sketches herself into a new page since it a was a very good idea to have her into the old Sketch Book that originally to her during her time as Izuku Midoriya.
Plus she is still trying to cope with the idea with her new personality and having no regard with killing a Villain Quirk or Quirkless. Even another part is being brutal toward the Villains.
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"Now that's done with it, I wonder what everyone was doing in my absents aside from just mourning me, but I can't really put up with Kacchan's bullying at me after learning I was Quirkless!" Likely Medusa hates Katsuki/Bakugo/Kacchan for bullying her during her time as Izuku Midoriya for being Quirkless.
"In his vocabulary, Quirkless people deserve to be look down, but he's just hot tempered."
Then she spots a paper flying in the air that's probably been blown in the wind and she catches then reads what this paper says, "Shigaraki AKA All For 1 and the Leader of the League of Villains." The paper she catches in her hands was actually a wanted poster and on it is Shigaraki AKA All for 1 and the Leader of the League of Villains, what happened next is a another new memory flashes into her head.
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"Oh so Shigaraki/All For 1 was not only the Leader of the League of Villains, he was also the arch nemesis of All Might", Medusa concluded that Shigaraki/All For 1 was also All Might's worst enemy.
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Elsewhere
Inko Midoriya, Izuku's Mother is visiting her son's grave, her husband Hisashi Midoriya is still away from Musutafu City but he was inform on what happened to their son leaving both of them with a lot of grief for their loss. When Inko arrive at her son's grave, she notice something was not right here, it's because she notice that Izuku's old Sketch Book is gone because she left it there to begin with and now it's gone, well she doesn't know that Medusa took it and she doesn't know that Medusa is really her son reborn and reincarnated.
Inko then proceeds to pick up her Cellphone and call none other than All Might, in the past she told All Might how much Izuku wanted to be a Hero and how much he idolizes All Might. Even the part of sadness that Izuku had after learning from the Doctor that he was Quirkless making him realize he won't be able to make his wish come true now.
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"Hello?"
"All Might I'm at my son's grave visiting but something is very wrong here."
"What happened?"
"My son's old Sketch Book is gone, you think you can help me find out what happened to it after done at the Unforeseen Simulation Joint?" But before she can get answer the line gets cut off for some reason.
"Hello All Might?" Inko doesn't know what happened and why she got cut off in the middle of her conversation, unknown to her Medusa passed by after jumping to the next rooftop.
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Unforeseen Simulation Joint UA High
What's going is while All Might and the rest of the Teachers of UA High were overseeing the Students undergo another field test when suddenly a Portal appears and a horde of Villains emerge from it including the League of Villains. All the Teachers who were Pro-Heroes even the young UA Students have spring into action for a defensive position against the League of Villains and the Nomu they brought with them.
So far many of them defeated the grunts of the League of Villains, Momo Yaoyorozu/Creati and Kyoka Jiro/Earphone Jack manage to defeat a large portion of them while defending their friend Denki/Chargebolt who just reach his limit after using too much power from his Quirk, Momo/Creati defeated much of them using a Metal Bo she created with her Quirk, Kyoka/Earphone Jack use her Earphone Jacks to defeat many of them.
Now they are all surrounded by many more League of Villain Grunts.
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"Oh great, we are cornered!" Momo/Creati said as she and Kyoka/Earphone Jack raise their hands as a surrender until they can come up something to get themselves out of this situation
"I supposed you at least have some sort of a plan?"
"I do but I just don't have the right moment to put it use yet Jiro."
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On another area All Might finish battling the Nomu that picks a fight with him and he knocks it out with a Alabama Smash before being confront by his worst enemy, Shigaraki/All For 1 then the 2 battle each other in the style of a titanic battle and try to finish what they started all these years ago. Unfortunately when All Might was putting up a good fight against Shigaraki/All For 1, suddenly that injury that All Might sustain years ago interrupts him causing him to weaken in the process. "No not now!" All Might falls to his knees weakened by his injuries while Nemuri Kayama/Midnight provides him with aid.
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"Bad timing isn't All Might! I destroyed your mentor Nana Shimura and now I'm going to destroy you and everything you loved in life!" Shigaraki/All For 1 prepares to kill All Might and Midnight, well Midnight knows she can't beat Shigaraki/All For 1.
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What happened next is coming at the Unforeseen Simulation Joint in high speed was Medusa who appears in a sparkling dust and tackles Shigaraki/All For 1 before he can deliver a finishing blow on All Might. "Who are you woman?!" Shigaraki/All For 1 gets back on his feet and demanded to know who she is but Medusa says nothing and slowly approaches him with her Duel Nameless Chain Daggers.
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Medusa attacks first in full speed and Shigaraki/All For 1 throws punches at her but miss and they resort to CQC but Medusa is so fast Shigaraki/All For 1 can barely keep with her movement and strikes until Medusa elbows Shigaraki/All For 1 on the face twice sending him flying before Medusa kicks him in the gut after appearing in front of him then stabs him in the left shoulder with one of her Nameless Chain Daggers making blood splatter, "you'll pay for that!" Shiragaki/All For 1 use one of his Multi-Quirks to throw sharp objects at Medusa but she evades them to the left and right before jumping high up into the air pulling Shigaraki/All For 1 who had one of Medusa's Nameless Chain Dagger attached to his left shoulder who then slams into a the sealing then into the simulated buildings.
Then Medusa spins him around slamming him into multiple objects like the Dome, Fountain before sending him flying and he crash into Simulated Mountain and skidding into the ground where he's fellow League of Villains come to his aid, oh the Nameless Chain Dagger already unattached itself from him when Medusa spin him around and send flying.
Shigaraki/All For 1 has been badly injured in his first fight with Medusa which was brutal.
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"You'll pay dearly for that woman, next time I'll becoming for you! From now on League of Villains it's time for us to retreat!" All of the League of Villains regroup and retreat from the Unforeseen of Simulation Joint because with their leader badly injured they need to retreat by entering the portal.
End
Now the UA Gang and All Might have seen Medusa and witness what she just did to Shigaraki/All For 1 in very violent and brutal way, they will be looking for her. Still they don't even know that she is there old friend Izuku Midoriya reborn and reincarnated. But they will be doing the pursue of Medusa inside the UA High Building.
There will be a Noble Phantasm for Medusa to use and the one for her to use is the one with the Pegasus which is known as the Bellerophon or Bridle of Chivalry. It will take some time for her to discover it and learn how to use against enemies.
I do like that fight with Medusa and Dark Assassin from the first Fate/Stay Night Heaven's Feel.