The Last Will and Testament of Albus Dumbledore

Harry doesn't know who to trust. It's been a year since his cousin, Percy, disappeared-again. And at the end of Harry's sixth year, Dumbledore is dead. Harry feels lost and alone...until he, Ron, and Hermione receive the contents of the Last Will and Testament of Albus Dumbledore. With mysterious clues all pointing towards the Son of Poseidon, Harry knows he will see his cousin again soon.

Sure enough, Percy shows up at the worst time, saves Harry's life, and joins the golden trio to hunt down the horcruxes and kill Voldemort.