After suffering his defeat at the hands of All Might and Nana Shimura, All For One had one last plan. Step 1: find and use Shimura's family, Step 2: the Nomu, Step 3: the ultimate weapon, a boy with the power of a monster.
All of the right to naruto...
We see a large man walking through a dark lab, passing large canisters as he goes. He moves towards a cell located in the corner of the room, finding a little boy no older than 3 or 4 curled into a ball.
AFO: Wake up boy.
As All for One opens the door the boy jumps up, standing at attention. the boy had shaggy black hair and was rather small for his age. This boy was created via an experiment done by AFO to combine an ancient beast and power stacking quirk into one, a sin to create a Sin. He had already created the other 6 and broken them to his will but the 7th had more pride than he anticipated.
AFO: I'm sending you to some colleague's to continue the experimentation, don't disappoint.
#7: Yes sir.
#7 was loaded into a van and taken to an underground location reminding him of a sort of maze. It was there he was met by a man wearing a medical mask of sorts.
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???: My name is Teiso Chizaki, and I'm here to complete your transformation into the Sin of Pride, the monster you were created to be.
All the sudden a huge beast was seen in the back of the room appearing to be a large fox with nine tails.
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Teiso: now that you already have your own quirk, I will fuse you and the 9 tails.
He grabs the boy before going into the beasts cage and fusing the two with his overhaul quirk. Afterwords looking towards his son Kai, and telling him that this is what their power is truly capable of. When #7 awoke he noticed his back had not 1 but now 2 different markings his old lion near the center of his back and lower shoulder blades and the new one just under it on his lower back.
