Voldemort's Request

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..."Hardly anyone knew of it at the time - I was one of the few in whom the then headmaster confided - but Voldemort first approached Professor Dippet and asked whether he could remain at Hogwarts as a teacher." "He wanted to stay here? Why?" asked Harry, more amazed still. "I believe he had several reasons, though he confided none of them to Professor Dippet," said Dumbledore. "Firstly, and very importantly, Voldemort was, I believe, more attached to this school than he has ever been to a person. Hogwarts was where he had been happiest; the first and only place he had felt at home." ... "Secondly, the castle is a stronghold of ancient magic. Undoubtedly Voldemort had penetrated many more of its secrets than most of the students who pass through the place, but he may have felt that there were still mysteries to unravel, stores of magic to tap. "And thirdly, as a teacher he would have had great power and influence over young witches and wizards. Perhaps he had gained the idea from Professor Slughorn, the teacher with whom he was on best terms, who had demonstrated how influential a role a teacher can play. I do not imagine for an instant that Voldemort envisaged spending the rest of his life at Hogwarts, but I do think that he saw it as a useful recruiting ground, and a place where he might begin to build himself an army."	Book 6, Pages 431-432 Half-Blood Prince, Chapter 20, "Lord Voldemort's Request"
"Let us speak openly. Why have you come here tonight, surrounded by henchmen, to request a job we both know you do not want?" Voldemort looked coldly surprised. "A job I do not want? On the contrary, Dumbledore, I want it very much." "Oh, you want to come back to Hogwarts, but you do not want to teach any more than you wanted to when you were eighteen. What is it you're after, Tom? Why not try an open request for once?" Voldemort sneered. "If you do not want to give me a job -" "Of course I don't," said Dumbledore. And I don't think for a moment you expected me to."	Book 6, Page 445 Half-Blood Prince, Chapter 20, "Lord Voldemort's Request"
"Do you think that's why he really wanted to come back to Hogwarts, sir?" said Harry. "To try and find something from one of the other founders?" "My thoughts precisely," said Dumbledore. "But unfortunately, that does not advance us much further, for he was turned away, or so I believe, without the chance to search the school."	Book 6, Page 505 Half-Blood Prince, Chapter 23, "Horcruxes"