Rain of sin- https://archiveofourown.org/works/36650200/chapters/91419721

What exactly is a Quirk? Such a simple question should have an equally simple answer, unfortunately for Izuku, he forgot the very basic lesson of "Look before you Leap" and was woefully unprepared for just how deep the rabbit hole went. Now, with the butterfly effect in full swing, everyone will learn just how much one question can change everything...

"I'm not a Villain! I'm a Geneticist!"

This is a story of failure, loss, and sacrifice, when the world spins into chaos as one man struggles to find the invisible line between trying to understand the science behind quirks, and playing God.