Princeps (Princeps, #1)

Harry has worked for years as an Unspeakable to identify the best point where he might go back in time to change the impact of Voldemort’s war. Now he knows: he will have to return to his parents’ Hogwarts years and encourage the Slytherins to stand on their own instead of following a leader. He knows how to assume the post of Defense professor and how to reach the Slytherins. And from there, well, surely nothing can go too wrong.

Fanfiction for Harry Potter by J. K. Rowling.