Detta behandlas i Korta berättelser från Hogwarts of Power, Politics and Pesky Poltergeists . I korthet blev Slughorn mer och mer störd av sin del i Voldemorts uppväxt till makten, speciellt hans berömda odödlighet. När det blev uppenbart att Dumbledore inte skulle stoppa i hans försök att förhöra honom om hans interaktion med Tom Riddle (och därmed avslöja sin mest skyldiga hemlighet, att han hade hjälpt honom att göra en Horcrux för att förhindra sin död) valde han att gå i pension , utvändigt hävdar misslyckande reflexer och 50 års lojal service till skolan.
When the wizarding world fell into war, and rumours swirled that Voldemort had, somehow, made himself immortal, Slughorn was sure that it was he who had made Voldemort invincible, by teaching him about Horcruxes (this guilt was misplaced, as Riddle already knew how to make a Horcrux, and had feigned innocence in order to find out what might happen if a wizard made more than one). Slughorn became ill with guilt and fright. Albus Dumbledore, now Headmaster, treated his colleague with particular kindness at this time, which had the paradoxical effect of increasing Slughorn’s guilt, reinforcing his determination never to tell a living soul what a dreadful mistake he had made.
Lord Voldemort made no attempt to seize Hogwarts on his first ascent to power. Slughorn believed, correctly, that he was safest remaining in his post rather than risking the outside world while Voldemort was at large. When Voldemort met his match upon attacking the infant Harry Potter, Slughorn was even more jubilant than most of the wizarding population. If Voldemort had been killed, Slughorn reasoned, then he could not have made a Horcrux, which meant that he, Slughorn, was innocent after all. It was Slughorn’s extremity of relief, and the disjointed phrases he let fall in the first rush of emotion after hearing of Voldemort’s defeat, that first alerted Dumbledore to the possibility that Slughorn had shared Dark secrets with Tom Riddle. Dumbledore’s gentle attempts to question Slughorn, however, caused him to clam up. A few days later, Slughorn (who had now completed a half century of service to the school) tendered his resignation.