Dumbledore Reveals the Memory is Tampered
Pages 371-372
After viewing the altered memory, Dumbledore explains that Horace Slughorn tampered with the memory because he is ashamed of what really happened. Dumbledore tasks Harry with persuading Slughorn to reveal the true memory, as it is crucial to understanding Lord Voldemort's plans and Mystery - Lord Voldemort's Horcruxes.
