The Curse of the Unicorn Blood

"...you will have but a half-life, a cursed life, from the moment the blood touches your lips." - Firenze. Hermione uses unicorn blood from 1st year detention to save Snape from Nagini's attack, causing them both to lead a cursed, half-life. Repeating 4th year at age 17. And what happens to the person who slayed the unicorn? Soul-bond, time travel, and more tropes. Non-canon, AU