Severus Snape - The One Who Walks in the Shadows
Severus passed away on the Shrieking Shack's floor only to find himself in the white space between the world of the living and the dead, conversing with one Master of Death Harry Potter.
He's offered the opportunity to return a few years back in time to try fixing a problem with the world and its magic, but also, his life.
He wants to better himself. He wants to be back in Lily's life. He wants to save magic.
Severus reluctantly accepts, going back to Hogwarts once again—as a student this time—to relive his worst memories.
However, this time he's not one of the chess pieces (easily manipulated, a target, one to be disposed of), no.
He's one of the big players, and he's ready to make his moves.
But also, why did James Potter become so obsessed with him? Hadn't Severus suffered enough in his past life? He's not ready for this crap.