About All For One and One For All!

"It's about the very origins of One For All," Toshinori started, and Izuku blinked.

"The... origin of One For All? Wasn't someone just born with it, like any other Quirk, only to find out later that it could be transferred?" Izuku asked.

"That's... partly true, but far, far from the full story," Toshinori said, before he started to go into a history lesson of sorts. He talked about the dawn of Quirks, and the chaos that followed. Then, he mentioned that one figure rose to prominence in the criminal underworld, and amassed power. Both personal, and influential. "His name... was All For One," Toshinori continued, taking a deep breath. "It was also the name of his Quirk. It gave him the ability to steal the Quirks of others, and then, either keep them for himself and his own use, or give them to others under his sway."

"He... Could take Quirks?" Izuku stared in shock.

"Yes. He became the most dangerous of all Villains through this Quirk," Toshinori nodded gravely, before continuing to explain just how much power he had, and how unstoppable he seemed. Then he talked about All For One's supposedly Quirkless brother who had opposed him. Until eventually, he had been forcibly given a Quirk by All For One, one that stockpiled power over time... Which merged with the Quirk said brother had had all along, one that allowed for the transfer of Quirks, useless at first, but then something with true potential when merging with the gifted Quirk.

He had them proceeded to pass along this Quirk from person to person, until, eventually, it had reached All Might. "I eventually faced All For One, and fought him, six years ago..."

"Six years... Wait, that was when..." Izuku realized, looking at All Might's left side.

All Might nodded, pulling up his shirt to show the still present, but much less prominent and more faded scar, left behind as the damage itself had been erased, as a permanent reminder. "Though I had defeated All For One, he had left me those grievous injuries," Toshinori admitted solemnly. "I had thought him dead, but... Given his body was never found, and recent events, I fear I am wrong about that."

Izuku paused, just processing everything.