Plot to smuggle diary; first hint to Horcrux

'No, no, no,' squeaked Dobby, shaking his head so hard his ears flapped. 'Harry Potter must stay where he is safe. He is too great, too good, to lose. If Harry Potter goes back to Hogwarts, he will be in mortal danger.'
'Why?' said Harry in surprise.
'There is a plot, Harry Potter. A plot to make most terrible things happen at Hogwarts School of Witchcraft and Wizardry this year.' whispered Dobby, suddenly trembling all over.
'Dobby has known it for months, sir. Harry Potter must not put himself in peril. He is too important, sir!'
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.
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'Albus Dumbledore is the greatest Headmaster Hogwarts has ever had. Dobby knows it, sir. Dobby has heard Dumbledore's powers rival those of He Who Must Not Be Named at the height of his strength. But sir,' Dobby's voice dropped to an urgent whisper, 'there are powers Dumbledore doesn't... powers no decent wizard...' (first hint to Horcrux)

DOBBY'S WARNING - Pg 16

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WHY LUCIUS MALFOY PLOTTED TO SMUGGLE TOM RIDDLE'S DIARY IN HOGWARTS THROUGH JINNY WEASLEY?

[Harry:] ‘But I thought he meant Lucius Malfoy to smuggle it [the diary] into Hogwarts?’

[Professor Dumbledore:] ‘Yes he did, years ago, when he was sure he would be able to create more Horcruxes, but still Lucius was supposed to wait for Voldemort's say-so, and he never received it, for Voldemort vanished shortly after giving him the diary. No doubt he thought that Lucius would not dare do anything with the Horcrux other than guard it carefully, but he was counting too much upon Lucius's fear of a master who had been gone for years and whom Lucius believed dead. Of course, Lucius did not know what the diary really was. I understand that Voldemort had told him the diary would cause the Chamber of Secrets to reopen, because it was cleverly enchanted. Had Lucius known he held a portion of his master's soul in his hands he would undoubtedly have treated it with more reverence – but instead he went ahead and carried out the old plan for his own ends: by planting the diary upon Arthur Weasley's daughter, he hoped to discredit Arthur, have me thrown out of Hogwarts and get rid of a highly incriminating object in one stroke. […]’

-Harry Potter and The Half Blood Prince