A little bit on the A/U side as it would be Hogwarts University, not a high school. Also, I would be giving Harry a twin sister. Harry, Hermione, Ron and Harry's twin sister Zelda would be returning to Hogwarts to finish their education. (Side note: Please do not ask me to change my role to Hermione or Ginny)
All seems well and peaceful in the wizarding world after Harry defeated Voldemort... or so they thought... a shred of a twenty year old Voldemort's soul was squirreled away by a previously unknown loyal follower and now, this follower has begun a series of profane dark rituals to use this shred to recreate Voldemort in flesh and blood as his twenty year old self. The catch is that Voldemort would remember nothing that happened before he was twenty.
After the success of their efforts, the follower begins to educate the dark lord on each and every thing that has happened over the last few decades.
A year later, Harry and everyone is getting ready to graduate from Hogwarts...
I thought maybe this would be a good way to go to bring Riddle and Zelda together... one of which was that Voldemort could pose as a transfer student from Durmstrang with intentions of getting revenge on Harry for his first incarnations downfall... and then he meets Zelda... and an obsession begins...
