A Study of Resonance

Percy Weasley is not particularly reckless, until he is, once, on the anniversary of his brother's death. For most people, the idea of sending oneself through time to correct one's mistake would remain a fantasy, something to dwell on in the middle of the night, but Percy has always been too capable for his own good.
Featuring: a reluctant spy, an unlikely friendship between a ghost and a lonely student, a portrait, very few friends, several very lengthy conversations about soul magic, and far too many Horcruxes.
(Or, an attempt to answer the question, if someone intelligent was sent back through time, armed only with the word 'Horcrux' and a general knowledge of the main timeline––not to mention a keen understanding of the Hogwarts curriculum––could that person logic his way to defeating the Dark Lord? Only time will tell).
COMPLETE.